Packages can specify where to find AndroidManifest.xml

By overriding LOCAL_MANIFEST_FILE, a package can now specify which
AndroidManifest.xml file to use. This also allows for AndroidManifest.xml to
be in a non-standard location, though that's not recommended.

Change-Id: Ib6d1baeef7601e5a93d4ce64d18883d45373fc12
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 75297bc..d841ac8 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -93,6 +93,7 @@
 LOCAL_NO_EMMA_COMPILE:=
 LOCAL_PROGUARD_ENABLED:= # '',optonly,full,custom
 LOCAL_PROGUARD_FLAGS:=
+LOCAL_MANIFEST_FILE:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.