[Connected devices page] Move the refresh logic to main thread.

As the caller of adding or removing devices may not be performing on background thread, sync problem could occur. So here we make sure all updates to UI happens on the main thread.

Test: atest: com.android.settings.connecteddevice.PreviouslyConnectedDevicePreferenceControllerTest
Bug: 365044575
Flag: EXEMPT bugfix
Change-Id: I5b697acb7cf7469fe3bbff33b820b4e9f4c5104f