I also took the opportunity to move a constant into
AndroidManifest.xml.in; it removes a layer of indirection when trying to
determine what Android package gets built.
The issue is that the preprocessed files need to be referenced as
$(CURDIR)/file to have Make handle the dependencies correctly. To
allow this, this patch adds ANDROID_MANIFEST_FILE (defaulting to
AndroidManifest.xml) and uses it in the appropriate places.
