RESTRICT AUTOMERGE Add test of startActivities with different uids

Ensure the activity with different uid cannot be the caller
of other the activities. That also means they won't be in
the same tasks.

Bug: 145669109
Test: StartActivityTests# \
      testStartActivitiesWithDiffUidNotInSameTask

Change-Id: I88e1d2fdaa43d6d2d13c49d967cc7b5da2cb3c4e