Supply app transition specs with a future

Because we retain activity surfaces now, the app transition specs
which were calculated/generated after the onPause() call when going
from recents -> app were too slow. Instead, supply a cross-process
future, which gets fetched when the window manager is about to be
ready to execute the app transition. In practice, this still gets
executed immediately after the onPause call.

If we have a retained surface, this adds some latency, but since we
absolutely need the specs to execute the transition, we have that
latency no matter where exactly we generate the specs.

If we don't have a retained surface, the specs are not calculated on
the critical path, so it's faster.

Bug: 19940527
Change-Id: I80d2c6f6b3a6568a70339619ecefbc3bd8409bd8