Skip reading config file that does not exist

By default there is no such file. This avoids printing
exception log on every boot. Especially it prints out
thousands of callstack when running unit tests.

Bug: 159102753
Test: LetterboxConfigurationPersisterTest
Change-Id: I64a62c4aa66ac045aa4b1ddf1ad2e51d7bf4ca5b
(cherry picked from commit 15942b75)
Merged-In: I64a62c4aa66ac045aa4b1ddf1ad2e51d7bf4ca5b