Define protection level for document manager.

Similar to what we've done for services like the installer, verifier,
and text classifier, we're creating a new permission protection level
for the "document manager" on the device, called the "documenter" in
this change.

There should be exactly one "documenter" on the device, since it
handles the sensitive MANAGE_DOCUMENTS permission.

Bug: 117745631
Test: atest CtsPermission2TestCases
Test: atest android.permission.cts.ProviderPermissionTest
Change-Id: I92c9fcfee24feae8dc3a7516cde093e8f3bf0e56