Fix external_primary volume assumption for picker mediaIds

The PhotoPickerProvider assumed that mediaIds are only for
external_primary volume. We can however have mediaIds for public
volumes. To resolve those, we use the synthetic 'external'
volume. This is fine because the operations are all read-only anyways.

Test: Manual
Bug: 227727273
Change-Id: Idd54a30b13c041674e62ed6db68fc112f3ac9749
1 file changed