Have RippleDrawable ignore non-attached RenderNodes

Don't try to do a RenderNodeAnimation if the RenderNode
isn't attached. Although this isn't strictly speaking a
valid state to be in, it's also easy for RippleDrawable
to just ignore it.

Bug: 186864959
Test: ripples still show up, are still RT accelerated normally
Change-Id: I8127f1419508157eb83ac9bb1562745ac53d2ced