Define splits for AUDIO/VIDEO/IMAGE permissions

Define three permission splits:

- READ_EXTERNAL_STORAGE to READ_MEDIA_AUDIO (targetSdk<T)
- READ_EXTERNAL_STORAGE to READ_MEDIA_VIDEO (targetSdk<T)
- READ_EXTERNAL_STORAGE to READ_MEDIA_IMAGE (targetSdk<T)

This results in the following behavior: any time any legacy application
(i.e., targetSdk<TIRAMISU) is granted READ_EXTERNAL_STORAGE, it will
also automatically be granted READ_MEDIA_AUDIO, READ_MEDIA_VIDEO, and
READ_MEDIA_IMAGE.

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

Bug: 201318587
Test: atest CtsPermissionTestCases CtsPermission3TestCases
Change-Id: I79171cbda6a3d6d1d4bb87994c8b02f7174ea4db