Added KCM for Serbian (Cyrillic).

It's used by two languages: Serbian and Montenegrin.

Also renamed keyboard_layout_serbian_latin.kcm to
keyboard_layout_serbian_and_montenegrin_latin.kcm.

Bug: 299861742.
Test: Flashed on device.
Change-Id: Ie2c08345a3c253302319b0da3e1c38e252f7f8d9