Add new Android.mk to handle repo move

We're moving the platform/build repository down a level, then symlinking
the directories and necessary files back into build/. So if we're still
in build/, keep searching for Android.mk files, otherwise stop, since
they'll be found through the symlinks.

Bug: 28001743
Change-Id: Ieea6e3b1fca265b548395c6af148ebb4efa43b0f