Fix DataChangedJournalTest.equals().

Problems:
 1. The class implemented equals() but not hashCode()
 2. equals() attempted to canonicalize the underlying path and silently
    returned false if the I/O operation failed.

Fixes:
 1. Implemented hashCode().
 2. Stopped equals() attempting canonicalization and built in directly
    on top of File.equals() instead. I also considered moving the
    canonicalization to the DataChangedJournal ctor, but that
     - had the undesirable side effect of stopping interaction with the
       journal for reasons other than the file itself being corrupt, and
     - posed the problem of what to do in case of an IOException during
       construction.
    The downside of course is that caller which (inappropriately) relied
    on equals() doing canonicalization can no longer rely on that. I
    didn't find any such callers.

Note:
 When DateChangedJournal.listJournals() fails to perform a directory
 listing (e.g. if the journal directory doesn't exist), it currently
 still returns an (empty) ArrayList<DateChangedJournal>. This CL does
 not address this, but in a follow-up CL I might change this to return
 an unmodifiable List or Set<DateChangedJournal>, and throw an
 IOException if the directory contents can't be listed.

Bug: 162022005
Test: atest FrameworksServicesTests:com.android.server.backup.DataChangedJournalTest

Change-Id: I94f35f9a3082f398758fe5b03fb8d260e4fb3e1b