Sub display list in TextView

TextView uses a sub-display list to 'cache' the rendering of its
text. This saves time when drawing an editable text, where the blinking
cursor forces a re-draw twice per second, which creates pauses during
scrolling.

Added a sub-display list invalidation when an appearance span is
modified/added/removed.

Also added an invalidation of the display list when selection range
is changed.

Change-Id: I41e8068a12902b8a745c5bb77de8c77def76a270