Android: Don't hard code the user dircetory path to /sdcard/dolphin-emu

Android is allowed to pick any path for the external storage media and
that's why we should always use Environment.getExternalStorageDirectory()
to get the external storage path

Source/Android/app/src/main/res/values/strings.xml

@@ -251,4 +251,6 @@
     <string name="load_settings">Loading Settings...</string>
 
     <string name="emulation_change_disc">Change Disc</string>
+
+    <string name="external_storage_not_mounted">The external storage needs to be available in order to use Dolphin</string>
 </resources>