Allow embedding activities cross apps

No longer limiting cross app activities being embedded because
the activities can originally be started into the task and
therefore no reason to prevent them from also being embedded.

Adding enforcement while embedding tasks. Only system or privileged
apps with ACTIVITY_EMBEDDING permission can embed tasks of
another app.

Bug: 189386461
Test: starting activities in embedded task

Change-Id: I6a7efe6357118da795b0432cbd9c6cf7d3db4b8f