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
diff --git a/recovery.cpp b/recovery.cpp
index ccb8e5d..a374eed 100644
--- a/recovery.cpp
+++ b/recovery.cpp
@@ -122,7 +122,7 @@
 static const int BATTERY_OK_PERCENTAGE = 20;
 static const int BATTERY_WITH_CHARGER_OK_PERCENTAGE = 15;
 static constexpr const char* RECOVERY_WIPE = "/etc/recovery.wipe";
-static constexpr const char* DEFAULT_LOCALE = "en_US";
+static constexpr const char* DEFAULT_LOCALE = "en-US";
 
 static std::string locale;
 static bool has_cache = false;