cmake: Detect OpenGLES/2 libraries on Android.

I don't know why cmake started not detecting OpenGL in my Android build
directory, maybe due to an upgrade.  Apparently Trent saw something like
that on his fresh Windows installation.  Detecting the libraries is the
right thing to do anyway.
Invert and replace ALLEGRO_CFG_NO_GLES2 by ALLEGRO_CFG_OPENGLES2.
android: Fix decodeBitmapStream signature at call site.

The signature was changed on the Java side in commit 157d408
but not reflected on the C side. --pw