Refactor SQLiteOpenHelper.

Combine the code for opening readable and writable databases.
This improves the handling of the case where a database cannot
be opened because it cannot be upgraded.  Previously we would
open the database twice: first read-write, then read-only, each
time failing due to the version check.  Now only open it once.

Removed the goofy locking logic related to upgrading a read-only
database to read-write.  We now do it in place by reopening the
necessary connections in the connection pool.

Change-Id: I6deca3fb90e43f4ccb944d4715307fd6fc3e1383