Add closeDatabase method to classes owning SQLiteOpenHelper instance

CTS often calls `pm clear com.android.documentsui` to clear the app's
local data, including the database file of LastAccessedProvider. In ARC,
this causes inconsistency between the actual database status and the
database status managed by the instance of LastAccessedProvider, because
DocumentsUI is persistent only in ARC and `pm clear` doesn't kill ARC's
DocumentsUI like it does in Android phones.

This CL introduces closeDatabase to the instances of SQLiteOpenHelper in
DocumentsUI. These will be called from ARC's DocumentsUI (ag/26552505)
to prevent the inconsistent state described above.

Bug: 304373253
Test: Same as ag/26552505.

Change-Id: I95ea5bc5aee91f0bf70b4ece2941d2f9007df84c