[RESTRICT AUTOMERGE] 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: 109817231
Change-Id: I057957e9b201c14dee185f9c00df157f178d8a31