Clean up potential busy files after key eviction.

There is a race condition between key eviction and killing user
processes.  The race condition is difficult to properly fix without
significantly degrading UI performance.

If the race condition occurs, decrypted filesystem data is left in
various kernel caches.  To mitigate, we try to ensure the caches are
flushed by evicting the keys again in a worker thread.

Test: Checked that the correct log messages appear when evicting a
user's keys
Bug: 140762419

Change-Id: I9e39e5bb0f5190284552bcd252b6213a22a51e91