AudioService: Add settings adapter for tests

AudioService currently calls many static methods to read/write settings,
which fail if triggered during unit tests. This CL introduces an adapter
for these methods, allowing more parts of AudioService to be tested.

This CL also grants the MODIFY_AUDIO_ROUTING permission to the tests, as
AudioServiceTest is currently failing without it. It is also required
for tests added in follow-up CLs.

Bug: 201781907
Test: make; atest AudioServiceTest
Change-Id: Ie438495309d14821824a88e6b68f7e6d9692b06b