Adds the basic Serbian files to LatinIME.

- Adds the Serbian keyboard layout.
- Adds the Serbian translatio to strings.xml.

Change-Id: I29739f422fd58d31fd2e5213b72908dd1f617805
Merge commit '8f3106e276ff04fde9f619f36b13c438589997bd' into oxygen_4.3

* commit '8f3106e276ff04fde9f619f36b13c438589997bd':
  Adds the basic Serbian files to LatinIME.