Start activity in the embedded TaskFragment if allowed

If the task already contains child TaskFragments, an activity
should be started in the child TaskFragments, but only doing so
if the activity is allowed to be embedded.

Currently, an app can embed its activities into its own task.
Only system or apps with ACTIVITY_EMBEDDING permission can embed
activities of another app.

Bug: 193456000
Bug: 189386461
Test: wm presubmit
Change-Id: I8707f5f79deb23cc9b2b8d647382416dce6a6e9d