Special handling of processes with recent tasks.

As the startup time of application processes has increased,
it is becoming more useful to try to keep around the process
for a recently used app, even if there are no longer any
actively running activities in it.  (For example, if you backed
out of the root activity.)

This change implements that behavior, keeping track of any recent
task entries that a process represents the root activity for,
and classififying them under a new cached proc state that is
managed the same as a cached activity.

Test: manual
Bug: 69386069
Change-Id: I430741646478763fd85b33a5384278ece831b2c3