Disable overriding task transit animation.

Activities can override transition animation when starting or finishing
activity by calling Activity#overridePendingAppTransition. This change
restricts it in a way such that an activity can do it only when the
transition happens within a same task.

Note that this change is guarded by a property, so by default,
activities can still override any transition animation. In order to
restrict it, the following property must be set:
persist.wm.disable_custom_task_animation

Bug: 162066454
Test: atest ActivityTransitionTests
Change-Id: I235adf73af75883d8e943b6f85d841a729f5bb6b