Text insertion cursor is now defined by a Drawable.

Bug 3261766

If defined, the drawable is used instead of directly drawing a 1 pixel
line. This makes the cursor more fancy and more visible.

The drawable is currently clipped by the TextView's limits, which is
currently visible on the left when the cursor is at the first position.
To solve this issue properly, we would need to propagate a do-not-clip
up in the hierarchy.

Change-Id: I99f6001048eed14104994acf6bab942dda8eb38e