audio policy manager: do not reapply same forced usage

Skip systematic audio path and volume reevaluation when the new
forced configuration for the selected usage is the same as current one.

Bug: 38276985
Test: try repro steps in b/38276985. Check regressions with audio routing for music
and calls over speaker, headset and Bluetooth

Test: connect BT A2DP and launch CNN
Change-Id: Id43a5270c666065e099d02d9e52de86b20cb8039