activity_manager_procstate_aidl: Create library for filegroup.

This is required by cameraservice_test_host. Other filegroups in this
.bp file have associated libraries, but none exists for
activity_manager_procstate_aidl.

Bug: 207411352
Test: Built libactivity_manager_procstate_aidl-cpp.
Change-Id: I24cd93271c83e825166f212e033ec5dbb05d8681