Extract common methods into ContentInterface.

Existing APIs that accept a ContentResolver are too restrictive when
the caller has their own ContentProviderClient already bound and
configured, so we're in the market for a solution to open those
existing APIs to accept a wider range of inputs.

The solution we've come up with is to introduce a super-interface
which contains the common ContentProvider APIs, and then make
ContentProvider, ContentResolver, and ContentProviderClient all
implement that interface for consistency.

After this change lands, we can then safely relax existing APIs to
accept this new ContentInterface, offering a clean path to solving
the problem outlined above.

Bug: 117635768
Test: atest android.content.cts
Test: atest android.provider.cts
Change-Id: Ic5ae08107f7dd3dd23dcaec2df40c16543e0d86e
Exempted-From-Owner-Approval: keep tests working