Gradle: Check for ANDROID_HOME

Uses the ANDROID_HOME environment variable to determine SDK location if
not specified
--sdkLocation is optional if ANDROID_HOME is specified
Falls back to sdkLocation if specified
Added missing LICENSE header to GdxSetupUI
Gradle: Allow fallback to sdkLocation if present
Merge pull request #1649 from Unkn0wn0ne/master

Gradle: Added ANDROID_HOME sdk check