app/proguard-rules.pro: Keep TreeDocumentFile constructor

We call it via reflection in our faster DocumentFile extensions and the
first parameter was being optimized out in release builds because it is
always null in the usages that R8 is able to analyze.

Related-to: #257
Fixes: #260

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

app/proguard-rules.pro

@@ -23,3 +23,9 @@
 # Disable obfuscation completely for BCR. As an open source project,
 # shrinking is the only goal of minification.
 -dontobfuscate
+
+# We construct TreeDocumentFile via reflection in DocumentFileExtensions
+# to speed up SAF performance when doing path lookups.
+-keepclassmembers class androidx.documentfile.provider.TreeDocumentFile {
+    <init>(androidx.documentfile.provider.DocumentFile, android.content.Context, android.net.Uri);
+}