Add media perms to DefaultPermissionGrantPolicy

For T, we recently added new permissions:

- READ_MEDIA_AUDIO
- READ_MEDIA_VIDEO
- READ_MEDIA_IMAGES

But, we failed to update DefaultPermissionGrantPolicy(+Test), and
therefore no pre-grants are being given for the new permissions. After
discussion, it was decided that any existing pre-grants for STORAGE
should be extended to these new permissions.

Bug: 229150830
Test: atest DefaultPermissionGrantPolicyTest
Change-Id: I7367c5981196b3c7988a835359b4e8d27b52b074