Fix LetterboxConfigurationPersisterTest when file is deleted

LetterboxConfigurationPersister can  use a different
file name for testing.

test_whenUpdatedWithNewValuesAndDeleted_valuesAreDefaults
has been fixed using two different persister instances
to avoid the problem of cached values.
Also DEFAULT_REACHABILITY_TEST  have been used as default
values to be different from reachability position constants.

Fixes: 290756407
Test: Run `atest WmTests:LetterboxConfigurationPersisterTest`
          `atest WmTests:LetterboxConfigurationTest`

Change-Id: I22388e0a6168b5bce2652a1d52da49c2188dd3c4