Clear out top activities not matching launched uid in task

This change is a continuation of ag/20482146. If we have task1 with
activities A1,B1, a task2 in the foreground could launch A2 with the
NEWTASK flag, and affinity for A1 - causing an activity sandwich we
wish to prevent. Hence, we clear out all activities not matching A's
UID when A2 is launched with NEW_TASK.

Test: atest ActivitySecurityModelTest
Test: manually tested on device
Bug: 248045984
Change-Id: Ia90b197aa7d41f91a7f8cc51b572cd344619c6ad