Fixes SecurityException when switching out of Guest user

The attached bug occurs when:
1. The main user has a screen saver QS tile installed
2. The guest user is switched to and a screen saver QS tile is also
   installed there
3. The guest user mode is exited and we return to the main user
4. QS is accessed

It happens because the DreamTile implementation didn't switch the user
ID on its UserSettingObservers. This CL adds that handling.

Fix: 362308685
Test: manually verified that, with this change, the steps above don't
reproduce the SecurityException crash
Flag: EXEMPT bug fix

Change-Id: I4e80719c64d40213989abfd1159234f7bab1bc2b