mcp: Synchronize mPendingStateRequest list access

Since this can be accessed either by MediaControlGattService
or by the MediaPlayerList as a result of media player state change
callback, it should be protected against concurrent access.

Bug: 204845801
Tag: #feature
Sponsor: jpawlowski@
Test: atest BluetoothInstrumentationTests
Change-Id: Ica3d6cd731d6e8946fb6ddfcc5a744f88d1c6044