Don't use merged manifest for android_library modules

Don't use the merged manifest for android_library modules.  We
still have to run manifest merger for android_library modules
because Make can't handle transitive dependencies, so it will
continue to merge the manifests at each library, and then merge
the manifests of direct dependencies into the final application.

Bug: 113294940
Test: m checkbuild
Change-Id: Ia8f9f910bd0a134730ddf2d542460eeddbc0a075