[Fix] Unable to show output device indicator after reboot

- To fix the NPE in MediaOutputIndicatorWorker
- To get Bluetooth device status in Slice, not in Worker
- To purify Worker to be an event receiver

Bug: 128945026
Test: make -j42 RunSettingsRoboTests
Change-Id: I9360202870c7a43a79641c5b84210c1bfad4b9a6
4 files changed