Allow starting activity for result on another TaskFragment

...but still in the same Task. This is only possible when
enabling ActivityEmbedding.

Meanwhile, the result should be sent immediately if the result-to
activity is in the RESUMED state.

Bug: 253387076
Test: atest TaskFragmentTest ActivityRecordTests
Change-Id: I9049e8cb2329836ac2661aa6ead679ffc838c3b8
Merged-In: I9049e8cb2329836ac2661aa6ead679ffc838c3b8