Editor: keep text selection when starting a process text intent.

This CL makes sure the selection is not cleared when a text processing
intent starts, so that we can properly replace the selection when it
comes back.

It also fixes a related issue with the selection handles. The previous
code assumed a text selection mode starts from touch and enters drag
mode by default. This CL makes the code conscious of whether it should
show the selection cursors or enter drag mode.

BUG: 20148456
Change-Id: Ie8632e4a96f431dc6b2e4d0b4d3978eb55149172