Log only a summary 'Found stale backup journal' message.

With the goal of reducing log spam, print only a summary 'Found stale
backup journal' messages instead of logging within the inner loop.
Previously, over 12k messages could be printed at a time from this
function.

Before this CL:
 - a backup was scheduled for each packageName from each stale
   journal
 - one (or two, if MORE_DEBUG) message was logged for each
   packageName in each journal file.

After this CL:

 - packageNames are de-duplicated before scheduling backups or logging
   (it's not clear to me whether duplicate packageNames previously
   occurred, in practice).
 - one message is logged for the number (if > 0) of stale journals.
 - one message is logged for the number (including their names, if
   MORE_DEBUG) of packages.

Bug: 161940947
Test: fewer 'Found state backup journal' messages printed
Change-Id: Ia1343e4cea31feb1eba9da561d20736eb5df0a14