Audio Policy: fix media over SCO logic

Fix the A2DP device override in Engine::getDevicesForStrategyInt()
which was ineffective as implemented.
Simplify Engine::isBtScoActive() to only retain the only condition
that matters: SCO is preferred for PHONE strategy.

Bug: 358120583
Bug: 292037886
Test: regression for phone calls over BT
Flag: com.android.media.audioserver.use_bt_sco_for_media
Change-Id: I6dabc2c2c9257ac2ef84593a0bd22b2b1c3b7a8f