Added overlay support for drawing/responding to text anchors.

Overlays let views draw and respond to touch events outside of their
bounds. This allows selection anchors to be friendlier and easier to
grab. This is currently private API, pending further evaluation.

Added themes/styles for text selection anchors.

Added assets for text selection anchors as provided by UX. The
left/right anchors are currently not suitable for use. They are here
for bookkeeping and replacement later. The theme currently uses the
'middle' anchor asset for all three. This will be changed once assets
are ready.

Change-Id: I01b21e5ae90cab201f86f38f2f5eeaf2bd7f6bcd