Fix RippleDrawable alpha

Alpha was being double-applied by being
both handled in the shader and applied
by the Paint. So change the Paint to not
apply the alpha since the shader does it.

Fixes: 272375156
Test: looked at ripples in dark & light theme, verified no visual change
Change-Id: I6ca4d32a7a5735ce8ec418014d0318c29dd3c8bf