Add <intent-filter> support to <provider>.

For the new documents work, we're only interested in the subset of
ContentProviders that actually implement DocumentsContract.  Instead
of returning all providers, add <intent-filter> support to make it
easier to limit the set of returned ProviderInfo.

Define a well-known action for DocumentsProviders, and start using it
when querying for roots.  Continue supporting the old <meta-data>
approach until all apps have been updated.

Bug: 8599233
Change-Id: I05f049bba21311f5421738002f99ee214447c909