Make starting window hw-accelerated

An earlier fix in L disabled hw acceleration for the starting window
after the system process became hw accelerated. This was done to preserve
the old behavior of the starting window having some default behavior
(in particular, being filled with a default color). However, this ends up
being a memory and performance problem on some platforms (memory because
some platforms have backing store for software surfaces, performance
because it takes far longer to create a screen-size software surface than
a hardware surface).

The fix is to allow the starting window to inherit the hw acceleration
behavior of its process, and to detect when we are drawing the contents
of that starting window and to fill it with a default color (black).

Issue #17443449 use hardware rendering for app preview window

Change-Id: I8be8111d9e38c51fbbc07185acca065815ce26dc