DataChangedJournal: Make nonfinal to partially fix a test breakage.

Context: KeyValueBackupTask's ctor takes a DataChangedJournal instance.
KeyValueBackupTaskTest uses a mock instance to check that remove() is
called. http://ag/12218310 made DataChangedJournal final, breaking the
test. This CL fixes the failure cause by making the class nonfinal.
Note that the test still fails because at least one unrelated failure
cause remains - see http://b/162022005#comment11

In the longer term, we may want to consider splitting DataChangedJournal
into an interface/abstract class + a concrete implementation; there's no
good reason why KeyValueBackupTask and KeyValueBackupReporter need to
depend on specifically the implementation class, when all they're calling
is remove() and toString(). However, the change to make the class final
wasn't important, and reverting it is the easiest / fasted way to unbreak
the test.

Bug: 162022005
Test: atest KeyValueBackupTaskTest

Change-Id: I22b2a5d65f03c4ccdcc718ed3866d37ba9441385