Prune files from /data/anr/ by number as well as age.

tombstoned prunes based on both age and number of files. Until we can
fully switch over to tombstoned, emulate that here too.

Bug: http://b/73140330
Test: ran tests

(cherry picked from commit 08de1891)

Change-Id: I6fd5dec94a379e53b45fd7e5a5f9fed2f211e5ac