screen_ui: Fix an issue in RTL locale detection.

The CL in [1] moved android.os.RecoverySystem to send the locale
argument in well-formed BCP 47 language tags (e.g. "en-US" instead of
"en_US"), with the matching changes to recovery code in [2]. However,
the one in ScreenRecoveryUI::SetLocale() was missed, which broke RTL
locale detection when using new format.

[1] commit 38715228 in platform/frameworks/base
[2] commit 2078b22e in platform/bootable/recovery

Test: Set the locale to "ar-EG". `Run graphics test` under recovery.
      Check the progress bar.
Test: Run recovery_unit_test on marlin.
Change-Id: I7c7f5e0725bfb096109c7192c19f3f008e8e47e3
diff --git a/screen_ui.cpp b/screen_ui.cpp
index 9198073..2a8d7f9 100644
--- a/screen_ui.cpp
+++ b/screen_ui.cpp
@@ -1099,9 +1099,9 @@
   rtl_locale_ = false;
 
   if (!new_locale.empty()) {
-    size_t underscore = new_locale.find('_');
-    // lang has the language prefix prior to '_', or full string if '_' doesn't exist.
-    std::string lang = new_locale.substr(0, underscore);
+    size_t separator = new_locale.find('-');
+    // lang has the language prefix prior to the separator, or full string if none exists.
+    std::string lang = new_locale.substr(0, separator);
 
     // A bit cheesy: keep an explicit list of supported RTL languages.
     if (lang == "ar" ||  // Arabic