Have Activity Transitions activited by a new window attribute.

Bug 17006497

Window content transitions were being enabled by default in
the Material Theme so that Activity Transitions could be
enabled by default. Unfortunately, this gave the effect that
many applications did not want -- the default transition between
window content is a fade out/in. Here, a new attribute is
added: windowActivityTransitions that is enabled by default
in the Material theme and windowContentTransitions is disabled
by default in all themes.

Change-Id: Iab453d608f00a48ff7ab7f09ce84b52cf5f20294