Add more specific start/stop dream reasons.

As dreams get more complex, we would like more specific loggings for how
dreams are started/stopped for better debugging.

This CL adds "reason" as a required parameter in the start/stop dream
internal/private API. This allows callers to specify reasons.

Bug: 252796782
Fix: 252796782
Test: verified reasons are correctly logged on device
Change-Id: Icea03eb6e23ca113f50fa047f6d57e6e1d0b5604