make AsyncStorage serially execute requests (facebook#18522)

Summary:
This patch is a bit of a hack job, but I'd argue it's necessary to dramatically improve the dev UX on Android devices. Somewhere in react-native, there's a shared SerialExecutor which AsyncStorage uses that is getting blocked, causing remote debugging to occasionally hang indefinitely for folks making AsyncStorage requests. This is frustrating from a dev UX perspective, and has persisted across several versions as far back as RN 0.44, and still remains on RN 0.54.

The issue seems to only happen on Android > 7+, which is likely because the ThreadPoolExecutor behavior changed in this version:
https://stackoverflow.com/questions/9654148/android-asynctask-threads-limits

Fixes facebook#14101

We've been using this patch in production for the past 4 months on our team by overriding the AsyncStorage native module. We use AsyncStorage extensively for offline data and caching.

You can test by compiling this commit version into a test react native repository that is set to build from source:

```sh
git clone https://github.com/dannycochran/react-native rnAsyncStorage
cd rnAsyncStorage
git checkout asyncStorage
cd ..
git clone https://github.com/dannycochran/asyncStorageTest
yarn install
cp -r ../rnAsyncStorage node_modules/react-native
react-native run-android
```

No documentation change is required.

facebook#16905

[Android] [BUGFIX] [AsyncStorage] - Fix AsyncStorage causing remote debugger to hang indefinitely.
Closes facebook#18522

Differential Revision: D8624088

fbshipit-source-id: 2a599fbca270a4826063cd93be8eaf796f34f57a

ReactAndroid/src/main/java/com/facebook/react/modules/storage/AsyncStorageModule.java

@@ -7,10 +7,13 @@
 
 package com.facebook.react.modules.storage;
 
+import java.util.ArrayDeque;
 import java.util.HashSet;
+import java.util.concurrent.Executor;
 
 import android.database.Cursor;
 import android.database.sqlite.SQLiteStatement;
+import android.os.AsyncTask;
 
 import com.facebook.common.logging.FLog;
 import com.facebook.react.bridge.Arguments;
@@ -43,6 +46,33 @@ public final class AsyncStorageModule
   private ReactDatabaseSupplier mReactDatabaseSupplier;
   private boolean mShuttingDown = false;
 
+  // Borrowed from https://android.googlesource.com/platform/frameworks/base.git/+/1488a3a19d4681a41fb45570c15e14d99db1cb66/core/java/android/os/AsyncTask.java#237
+  private static class SerialExecutor implements Executor {
+    final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
+    Runnable mActive;
+    public synchronized void execute(final Runnable r) {
+      mTasks.offer(new Runnable() {
+        public void run() {
+          try {
+            r.run();
+          } finally {
+            scheduleNext();
+          }
+        }
+      });
+      if (mActive == null) {
+        scheduleNext();
+      }
+    }
+    protected synchronized void scheduleNext() {
+      if ((mActive = mTasks.poll()) != null) {
+        AsyncTask.THREAD_POOL_EXECUTOR.execute(mActive);
+      }
+    }
+  }
+
+  private static final Executor ASYNC_STORAGE_EXECUTOR = new SerialExecutor();
+
   public AsyncStorageModule(ReactApplicationContext reactContext) {
     super(reactContext);
     mReactDatabaseSupplier = ReactDatabaseSupplier.getInstance(reactContext);
@@ -141,7 +171,7 @@ protected void doInBackgroundGuarded(Void... params) {
 
         callback.invoke(null, data);
       }
-    }.execute();
+    }.executeOnExecutor(ASYNC_STORAGE_EXECUTOR);
   }
 
   /**
@@ -208,7 +238,7 @@ protected void doInBackgroundGuarded(Void... params) {
           callback.invoke();
         }
       }
-    }.execute();
+    }.executeOnExecutor(ASYNC_STORAGE_EXECUTOR);
   }
 
   /**
@@ -259,7 +289,7 @@ protected void doInBackgroundGuarded(Void... params) {
           callback.invoke();
         }
       }
-    }.execute();
+    }.executeOnExecutor(ASYNC_STORAGE_EXECUTOR);
   }
 
   /**
@@ -322,7 +352,7 @@ protected void doInBackgroundGuarded(Void... params) {
           callback.invoke();
         }
       }
-    }.execute();
+    }.executeOnExecutor(ASYNC_STORAGE_EXECUTOR);
   }
 
   /**
@@ -345,7 +375,7 @@ protected void doInBackgroundGuarded(Void... params) {
           callback.invoke(AsyncStorageErrorUtil.getError(null, e.getMessage()));
         }
       }
-    }.execute();
+    }.executeOnExecutor(ASYNC_STORAGE_EXECUTOR);
   }
 
   /**
@@ -379,7 +409,7 @@ protected void doInBackgroundGuarded(Void... params) {
         }
         callback.invoke(null, data);
       }
-    }.execute();
+    }.executeOnExecutor(ASYNC_STORAGE_EXECUTOR);
   }
 
   /**