Conditionally set ANDROID_HOME only if not set elsewhere
Conditionally set ANDROID_HOME only if not set elsewhere

I think we can use the plugin's SDK copy after all.  Let's set
ANDROID_HOME conditionally so that we don't override an existing value set
elsewhere.

Change-Id: Id02a47f12be577d349c0e99896261c3255f2dc24

dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia,wikimedia/apps-android-wikipedia,wikimedia/apps-android-wikipedia,wikimedia/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia