Add the missing sr-Latn into png files and rename the png locale header

Switch the locale header in the png files from Locale.toString() to
Locale.toLanguageTag(). For example, en_US --> en-us and sr__#Latn
--> sr-Latn. Also clean up recovery a bit to expect the new locale
format.

Bug: 35215015
Test: sr-Latn shows correctly under graphic tests && recovery tests pass
Change-Id: Ic62bab7756cdc6e5f98f26076f7c2dd046f811db