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
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 4224fda..1a33153 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -469,6 +469,8 @@
 LOCAL_CUSTOM_BUILD_STEP_OUTPUT:=
 LOCAL_IS_AUX_MODULE :=
 
+full_android_manifest :=
+
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.
 # Leave the current makefile to make sure we don't break anything