AVRCP Controller request focus when idle

When no music is playing and an incoming play request comes in from a
paired device, request focus on it's behalf.

Bug: 135112399
Test: atest com.android.bluetooth.avrcpcontroller.AvrcpControllerStateMachineTest
Change-Id: Ia764ea54af95166be6b6c3d9b47f3e3c2d09a0d7