Pass a bg handler when registering broadcast receiver

This should hopefully ensure that we're avoiding doing binder calls on
the main thread.

I made it nullable because we don't have a great way to fake a handler
in SettingsLib, unless we want to add a dependency on
SystemUITestUtils...

Bug: 348810424
Test: ZenModeRepositoryTest
Flag: com.android.settingslib.flags.volume_panel_broadcast_fix
Change-Id: I6aa1e45efa08dcd7e3352db3b383d317d1a56b90