Public APIs for ID-specific Uris, misc items.

We have getContentUri() for entire collections of items, but we
only have ID-specific overloads for some of the MediaStore classes;
let's get them all added for consistency.

Remove primary/secondary directory logic, which was replaced by
new RELATIVE_PATH column before Q launched.

Bug: 137890034
Test: atest --test-mapping packages/providers/MediaProvider
Exempt-From-Owner-Approval: trivial API refactoring
Change-Id: Iae4e7fe57adff071c35af459e31223a1fd05fef2