Add startDreamActivity request verification

The ActivityTaskManagerTestService has a special method to start a
DreamActivity. This CL adds a verification check before the activity is
started to check that the caller is the currently active dream
component.

Bug: 133216167

Test: atest DreamManagerServiceTest

Merged-In: Id71b4cbc57c6569f58b6d906cc8361c104a507dc
Change-Id: Id71b4cbc57c6569f58b6d906cc8361c104a507dc
(cherry picked from commit 20dc5182)