Workaround fading edge incompatibility with alpha modulation

bug:11121809

Fading edges, such as drawn by a scrolling marquee TextView, don't
behave correctly when their view is modulated, as done by the view
system when alpha is set on a view that returns false from
hasOverlappingRendering.

To work around this, always return true from hasOverlappingRendering
if a TextView has horizontal fading edges enabled.

Change-Id: I64c9b8c653aa1d9ca320292306fd24333e3ee225