Send instances creation in media device manager to bg thread.

This change also execute all entries operations in background thread
since it is not thread safe.

Flag: com.android.systemui.media_controls_device_manager_background_execution
Bug: 400200474
Test: atest MediaDeviceManagerTest

Change-Id: I3b8dbc38403c3fb9a052d899749051f60c6bdf0c