ActivityManager: support new free memory estimate

Add support for a new free memory estimate which
instead of using the PSS of cached apps uses the
private dirty and private clean memory of cached
apps to estimate their size.

This new free memory estimate is more accurate
because it doesn't count the shared memory used by
cached apps which doesn't tend to be returned to
the system when cached apps are killed.

The new free memory estimate can be seen by using
the -f option with the dumpsys meminfo command.

Change-Id: I11d326f7937af79b7e909cbfa5eed568f26f7b34