AudioService: cache routing queries by AudioAttributes

Add support for caching the result of getDevicesForAttributes
in the AudioSystem adapter class, as well as stats and
debug mode.

Bug: 162448412
Test: atest com.google.android.gts.audioservice.AudioServiceHostTest#testDevicesForAttributes
Change-Id: I7b45fd0c4c6f2f00c1c3bf6fde02dbf5624c91d8