Rename READ_MEDIA_IMAGE to READ_MEDIA_IMAGES

This permission was only created very recently, and it is unlikely that
any apps are using it yet.

After its creation, it was decided that it should be renamed.

Bug: 223691971
Test: atest CtsPermissionTestCases CtsPermission3TestCases
Change-Id: If82cee5995a2adc8e1b1fcc88ea7b540e7df0fac