Treat IME processes as hosting activities

When we stop using an IME process, we still treat that
process as if it is hosting activities (in the activity part
of the LRU list), to try to keep it around.  This is intended
to help the experience of switching between IMEs, reducing
the chance of your previous IME process being killed and thus
requiring much more time to switch.

Change-Id: Ie5793fd9b40d980fa18f80246326511ed6ae0597