Register observers to update the summary on ScreenSaverScreen

In screen saver preference, register observers for the dream enabled setting
or the active dream selection setting.
So the screen saver preference subtitle can be updated to reflect the new state.

NO_IFTTT=Catalyst only

Fixes: 413277054
Test: on device
Test: atest SettingsUnitTests:ScreensaverTest
Flag: com.android.settings.flags.catalyst_screensaver
Change-Id: Ibe160917ec4692d98aae8b38e4ec748229717f78