android: Use Storage Access Framework to import certificates

Thanks to the SAF, introduced with Android 4.4, browsing and opening
files on the system is very easy to implement.

On older systems the menu option is removed.

src/frontends/android/res/menu/certificates.xml

@@ -15,6 +15,11 @@
 -->
 <menu xmlns:android="http://schemas.android.com/apk/res/android" >
 
+    <item
+        android:id="@+id/menu_import_certificate"
+        android:title="@string/import_certificate"
+        android:showAsAction="withText" />
+
     <item
         android:id="@+id/menu_reload_certs"
         android:title="@string/reload_trusted_certs"