Don't recompute location of AndroidManifest.xml

The location of AndroidManifest.xml has already been computed
by android_manifest.mk, don't duplicate the logic.  Fixes an
extra regeneration triggered by $(wildcard $(my_android_manifest))
when LOCAL_FULL_MANIFEST_FILE points to a generated file, which
will not exist on the first run of kati.

Test: m
Change-Id: Ia56693a4dbbc7d78900019b887f766938dc55dab