ANDROID_SDK_ROOT environment variable support

[brody4hire]

As reported by @vtaliar in apache/cordova#68:

According to [1] use of the ANDROID_HOME is deprecated and we should use ANDROID_SDK_ROOT instead.

I would favor a quick fix which would use ANDROID_SDK_ROOT if ANDROID_HOME is not defined or does not point to a valid Android SDK directory, to reflect behavior described in [1].

I would also like to see this fixed before we issue the next major release for Cordova 9 as discussed in apache/cordova#10.

P.S. Current behavior is documented in [2].

[1] https://developer.android.com/studio/command-line/variables
[2] https://cordova.apache.org/docs/en/latest/guide/platforms/android/