Fix GestureOverlayView handling

Fix GestureOverlayView handling

This CL fixes a number of problems related to the GestureOverlayView.

The first big problem was that GestureOverlayView lives in the
android.gesture package, which is not one of the special builtin
packages you can omit when specifying the element in layout XML
files. This CL both stores a full path in the descriptor XML name,
which fixes tag-to-descriptor lookup (without this we were showing the
wrong icon and missing vital attributes in completion), and handles
scenarios where the full path has to be used.

The second problem was that the descriptor metadata listed
GestureOverlayView as a "view" rather than a "layout", which meant
that it was missing information about children (which meant you
couldn't drop into it), and it was missing layout params (which meant
children couldn't be assigned attributes like layout_width and
layout_height).

Also tweak selection handling for gesture views.

There are a number of places where we need to go from a class name to
a view descriptor; these are now centralized in a utility method and
sped up with a map lookup.

(Issues fixed: #17541, #17543)

Change-Id: I67c450f28eab07b3575e510420b7faf4410085e3