Remove libSR_AudioIn from base.mk

This is an obsolete rule that was grandfathered in because
it was a USER module at some point. It's no longer
required, even by builds that use packages/app/VoiceDialer.

Change-Id: Ife9e89bd1b03c0364e27650863a83bad945b8089
Simplify the calculation of LOCAL_UNSTRIPPED_PATH

Also this fixes the LOCAL_UNSTRIPPED_PATH if the module is installed to
the vendor dir via LOCAL_PROPRIETARY_MODULE.

Bug: 11289169
Change-Id: Ib07e5761411210963076487fe0e148c259e1e082