Improved Serbian translation

app/src/main/res/values-sr/strings.xml

app/src/main/res/values-sr/strings_contributors.xml

app/src/main/res/values-sr/strings_defaults.xml

app/src/main/res/values-sr/strings_error.xml

app/src/main/res/values-sr/strings_format.xml

app/src/main/res/values-sr/strings_info.xml

app/src/main/res/values-sr/strings_messages.xml

app/src/main/res/values-sr/strings_preferences.xml

app/src/main/res/values-sr/strings_settings.xml

app/src/main/res/values-sr/strings_setup.xml

app/src/main/res/values-sr/strings_status.xml