Multiple improvements and fixes in Android build process

Fix verification of Android SDK path

$ANDROID_SDK/tools/zipalign no longer exists, so it is a better
idea to check for $ANDROID_SDK/tools/android.

tools/android/packaging/Makefile: Fix appt, dx and zipalign paths

These executables have been moved to the build tools.

Get build tools paths in configure rather than in Makefile

Check $ANDROID_HOME/platform-tools for build tools too

Check $ANDROID_HOME/tools for build tools too