Clear Binder identity on calls into DreamService.

There are multiple entry points where DreamService is accessed
by the either the System or SystemUI. In these cases, the
Binder identity should be cleared to prevent the DreamService
implementation from running in an elevated state.

Test: atest DreamServiceTest
Fixed: 406895224
Flag: EXEMPT bugfix
Change-Id: I8e76f590d6f65ae689f5220b233b7197f2e731b4