Some small tweaks to improve memory management.

We now allow processes that currently have stopping activities to
be managed as if they were done stopping, so that memory trimming
can be done before the process goes to the background.  Hopefully
this will reduce cases where the processes goes to the background
and immediately gets killed, but wouldn't have had to be killed if
it had a chance to trim its memory.

Also change window memory trimming to always do the aggressive
trimming when memory is critical, even if not on a low-end device.

And tweak web view trimming to not trim for foreground UI events.

Change-Id: I241b3152b52d09757bd14a202477cf69c9b78786