Convert libsoundtriggerservice to Android.bp

See build/soong/README.md for more information.

Removed AUDIOSERVER_MULTILIB because it should be unnecessary -- almost
every device does not set it, and the only other value is "64" for
64-bit only devices, which will act the same as "" for libraries.

Test: cd frameworks/av; mma
Change-Id: I8f62a549bcd5ea9eaf9aa6ead48ca6748a31c873