Map "sr-Latn" to "sr_ZZ" for internal use.

We want to expose Serbian (Latin) layout as "sr-Latn" to the system,
while our internal logic may not be ready to deal with "sr-Latn" yet.

As a temporary workaround, we remap "sr-Latn" into "sr_ZZ" for our
internal use.

Bug: 27348943
Change-Id: I93ff0c75b3687bb1b913f451b9eb5f2820beefbc