Reduce number of saveLayer calls in RippleDrawable

Removes an extra saveLayer call by rendering ripples as overlapping. We
are now down to zero saveLayers when drawing unmasked ripples or ripples
masked against opaque content/masks, one saveLayer for ripples masked
against content and two saveLayers when masked against an explicit mask
layer.

BUG: 18226391
Change-Id: I0fc09d21fbc462fbcfe4c26fc7b18737f584043e