Defer pausing apps when bringing up recents

This adds an ActivityOption call "transient launch" which can be
used to tell WM that this particular launch is likely temporary
and may not be committed. This is for recents where it exists
for the duration of the transition, but then disappears once the
user's intention is resolved.

The effect of this is that it won't pause the running app while
in recents -- instead deferring pause until the transition finishes
and the final visibility states are committed.

Bug: 183993884
Test: enable shell transitions, open app, swipe to overview and
      check dumpsys to see that the app is still RESUMED
Change-Id: Iba6b8faf4e42c645bfd0305b93b7e0b542fd2c96