Multiple display lists for editable text

Bug 5763685

Long text in a ScrollView (not when the View's internal
scroll is used) is cached as a unique display list when hardware
rendering is on.

As a result, each time the text is edited, the entire display
list has to be updated, which takes a significant amount of
time (up to 500ms for a few thousand lines), proportional to the
size of the text.

This CL splits the text into multiple display lists as the
text is edited. The boundaries of the display list are aligned
with paragraphs.

There is still an issue when the number of lines changes: onLayout()
is called which invalidates all the display list. When the source
of that change is line wrapping and not a change in the view's
dimensions, we should be able to simply shift down the previous DL
instead of re-creating everything.

Change-Id: I7de49a1e5637cdfc9ef06b64b1ec4b61d9ea2415