Create separated tasks for different apps from startActivities

Assume there are 2 applications A, B with different uids.
There are 4 activities A1, A2, B1, B2 with default task
affinity and launch mode.

After A1 called startActivities(B1, A2, B2):
 Original   : Task(A1, B1, A2, B2)
 This Change: Task(A1, B1), Task(A2, B2)
In other words, the source caller cannot launch its activity
above the activity of other application in the same task, and
it can still launch activity of other application in its task.

Bug: 145669109
Test: atest StartActivityTests# \
      testStartActivitiesWithDiffUidNotInSameTask
Change-Id: I97bd875146a52f62b8fe82235487ccefb2955e8e