Simplify ripple background drawing, fix ripple alphas

Eliminates an extra saveLayer on the background in the common case of
a rectangle-bounded ripple.

Ripples and backgrounds are now drawn at 50% opacity of the ripple
color, which ensures that both the ripple and background are visible
and that the pressed state has a correct combined alpha.

Also fixes a bug where hardware (RT) animation was getting turned off
prematurely.

BUG: 17405007
BUG: 17398089
BUG: 17394445
BUG: 17389859
Change-Id: Idb5808368fe563581a51a8cb9778275ee8d22f4c