Avoid redundant AdditionalSubtypeMapRepository#putAndSave()

This is a follow up CL to my previous CLs [1][2][3], which aimed to
fix bugs that additional subtypes were cleared only when the current
user's IME APKs were updated but not for other users'.

What this CL does it to avoid unnecessary file operations by
making sure that

  AdditionalSubtypeMapRepository#putAndSave()

gets called only once from

  MyPackageMonitor#onFinishPackageChangesInternal().

This guarantees that for each package callback transactions additional
subtype persistent file will be saved at most once, compared to
multiple times in the previous implementation. Hopefully this helps
mitivate the potential performance regression discussed in Bug
327861441.

The observable behavior should remain the same semantically.

 [1]: I07e32739f486d960c9dd22476120fa35bf1899e4
      0c5feb22
 [2]: I10aa547f0de607ef3c0ba26764dac0585c40c843
      2501e749
 [3]: If42c518765171ff8cb51af000542671676cd3801
      87218eb2

Bug: 309837937
Bug: 322062773
Bug: 327861441
Bug: 328098968
Test: presubmit
Change-Id: I5b3dae7579feb412517251c6c8ad770b49831600