A2dpService: Test: use static flagging

static flagging is the recommended way to use trunk stable flagging
Change binder builder to take dependency in argument at construct and
not during its runtime

Bug: 321252288
Bug: 300174072
Test: atest A2dpServiceTest
Test: atest A2dpServiceBinderTest
Flag: Exempt, Test only
Change-Id: I9750f09c263cc269eb097a6fa3e3244b66e27401