Add method to Activity to allow cross uid launches

As part of go/activity-security, we will begin blocking launches which
do not match the current top Uid in a stack.

Thic change introduces a way for activities to opt out of the new
restrictions, by calling setAllowActivityTransitionFromBelow(true) at any
point in their life cycle.

Bug: 262567954
Test: atest ActivitySecurityModelTest ActivitySecurityModelEmbeddingTest
BackgroundActivityLaunchTes

Change-Id: Ie20f95d0fcf94f50b128b5a7a0553ac02e0fbdc6