Grant apps with WRITE_MEDIA_STORAGE legacy storage appop

An app can now have legacy storage access in 3 mutually exclusive
ways:
1. Targeting < Q
2. Targeting <=Q and explicitly requesting legacy access
3. Targeting <=Q and having the WRITE_MEDIA_STORAGE permission

In other words an app targeting R has no way to have legacy storage
access. Its only option is to request MANAGE_EXTERNAL_STORAGE access

Test: Verify via dumpsys the conditions above
Bug: 144914977
Change-Id: I3c868cd9b4da4083517dd64b3a3888aea4060f18