Fix issue #16311398: Limit number of documents a process can open

In application processes, monitor for when we start getting close
to the Dalvik heap limit, and ask the activity manager to try to
prune old activity instances in that case.

Add an explicit API for apps to ask that they have their own
activity instances cleaned up, if they want.

Fix some bugs in launching activities that were not correctly
applying the "multi task" behavior in the appropriate situations
of document-centric recents.

Clean up the activity manager's process removal code to all share
a common path.

Add a new "Spam" option to ActivityTests, which continually creates
new tasks, checking that the activity manager will now prune old
tasks rather than letting the app run out of RAM.

And while I was was doing this, I found problems with the path
for bringing an empty task to the foreground -- it could make
a new task instead of re-starting the root activity in the
existing task.  This is fixed, and some code in the recents
UI for working around the bug is removed.

And as long as I am doing that, we now have nice hooks in to
the activity manager for AppTask to give some APIs for better
managing the task, so add those along with more tests for these
APIs in ActivityTests.

We should look at also having the activity manager try to prune
old tasks when it sees app processes being killed, to better balance
memory use across multiple processes when some processes may host
many documents.  That however is for another CL...

Change-Id: I2bb81c3f92819350c868c7a7470b35817eb9bea9