Clear some aapt2.mk inputs after use

my_generated_res_dirs wasn't being reset in some cases in between
different modules. To be safe, clear every input variable that I was sure
was not being used by the caller after aapt2.mk.

Test: m
Test: m ANDROID_BUILDSPEC=vendor/google/build/app_build_spec.mk
Change-Id: I37c861d99637cad77a2a378a119bf66cee373b3a