Fixes quick switch animations for low end devices

Currently using quick switch is fine but sometimes with animation
glitches but very reproducible if tapping recents button very fast. The
problem is that low end device Recents entrance animation also has the
previous tasks animating downward causing more chances of animation
instability.

The problem is that transition is not finished and it is already
starting to do the next toggled animation. When the transition from
app to recents (RecentsImpl.toggleRecents) starts without finishing
RecentsActivity.onStop, the previous frame will be visible for a couple
of frames before the entrance animation starts (having the previous task
animate down). Therefore restrict toggling to after
RecentsActivity.onStop would allow the previous frame to finish and the
transitions run normally.

Bug: 62251652
Fixes: 64401391
Test: manual - using gobo device, launch multiple apps, rapidly tap recents
button
Change-Id: I4e70434bca3c9bec287fa30559b23a1e71b5ef20