Define new AUDIO/VIDEO/IMAGE permissions

Define three new permissions:

- READ_MEDIA_AUDIO: read audio files from external storage
- READ_MEDIA_VIDEO: read video files from external storage
- READ_MEDIA_IMAGE: read image files from external storage

Also, define two new permission groups:

- READ_MEDIA_AURAL
- READ_MEDIA_VISUAL

The permissions are assigned to permission groups as follows:

  READ_MEDIA_AURAL {
      READ_MEDIA_AUDIO
  }
  READ_MEDIA_VISUAL {
      READ_MEDIA_VIDEO
      READ_MEDIA_IMAGE
  }

The motivation for this change is to support the "Media Permission
Update for Android T" project.

Bug: 201318587
Test: manual
Change-Id: If3d357baed15b66319a6c2308416622e5e57fe28