Optimize hot launching recents

Rearrange how we generate the transition specs, which involves
creating a thumbnail on the mainthread (about 10ms on large
devices): First, we put launching the activity onto a handler
thread (with default priority), to free up the main thread. Then,
we immediately start generating the thumbnail such that when the
future calls us we have the generated spec already handy.

For that we need to be able to supply a specs future into
ActivityOptions, to avoid race conditions. Furthermore we need to
make sure not to call into WM while creating specs, to avoid WM
lock contention.

Test: App -> Recents -> Same app, inspect app transition logs
Test: Double tap recents for quick switching

Bug: 32668632
Change-Id: Ic6ec65c2560f67cade3b5ddde9f79ee13e9ba32c