Fix warning with AAPT2 and LOCAL_STATIC_ANDROID_LIBRARIES

Building with LOCAL_STATIC_ANDROID_LIBARIES and LOCAL_USE_APPT2
causes a warning:
build/core/package_internal.mk:143: Empty argument supplied to find-subdir-assets

Only call find-subdir-assets if my_res_dir is not empty.

Also improve the warning message to make it easier to find the module
that caused it.

Test: m -j
Change-Id: I9a71162c7e2ed82f64d6844baca256968ac77317