Ensure mDreamToken is only accessed on main thread.

This changelist updates access to mDreamToken to only happen on the
main thread. Most calls already happen this way for view and
lifecycle callbacks. However, a number of calls happen from other
invocations such as binder calls or invocations from DreamService
implementations. In such cases, the logic should be marshalled
onto the handler thread.

Fixes: 382609257
Flag: EXEMPT bugfix
Test: atest DreamServiceTest
Change-Id: Ifa142368838e7330ae042480d34bed0c23184894