Set alpha channel correctly when recomputing gradient

The alpha channel is applied by modulateAlpha(). Setting it to
mAlpha in ensureValidRect() locked the maximum alpha channel to
whatever value was set before layout. Setting it to BLACK
ensures it can be modulated later.

BUG: 10514848
Change-Id: If7a824410a66f1e9b648e20ee627bdf99a42ae78
1 file changed