Fix bug 3066313 - integrate new text anchor assets.

This adds the differentiated left/right anchors for hdpi and further
tweaks the behavior of text anchors.

Highlights:
* New assets are translucent - increased visibility for whatever is
  beneath.
* Start and end anchors may not cross one another.
* Minimum selection size is now 1.
* Touch targeting tweaked.

Change-Id: I601361323916ee19149562a23dc3242d908da45c