Replace system gallery permission granting with system gallery role.

This change also adds a system API config_systemGallery, because the
old way of determining the system gallery app by
Intent.CATEGORY_APP_GALLERY is considered unreliable.

Bug: 144307267
Bug: 136500975
Test: manually confirm that the system default gallery is granted
      privileges

Change-Id: I8b1bf79e9041108ed5253fd6c89c9ee8e78faac8