Enable activity rendering during window animations

A change was made back in ICS that prevents the view hierarchy from rendering
during window animations. Specifically, it allows the hierarchy to render once (to draw
the results of its first layout), but further drawing is suppressed at the
ViewRoot/performTraversals level until the window animation is complete.

This change was introduced to avoid jank problems that were resulting from
thrashing the GPU by issuing drawing commands from multiple processes simultaneously,
and limited the number of rendering processes to mainly the system server (and
possibly the System UI), which allowed window animations to be much smoother.

This fix contributed to another source of jank, however, in which applications
which attempt to animate when they first appear will not render any frames of
animations until the window animation is done, resulting is a snapping to the resulting
state once the window animations are complete.

Meanwhile, hardware has gotten faster and GPUs have gotten better, and it is time to
revisit this logic. This change disables the earlier fix and allows view hierarchies
to draw normally, regardless of whether window animations are taking place.

Issue #22232939 Remove flag that prevents drawing during window animations

Change-Id: I4c960180771ff09a7088abd77b437586e835a991