when corruption occurs, log a warning before closing db

and database.close() should NOT set mPath to null.

a few other minor changes included in this CL
1. if it is memory database, no need to delete any file
2. if 2 threads are sharing the same connection, and if corruption occurs
on it, one thread closes the db and deletes db - while the other
thread is still using it. this can cause SQLITE_MISUSE error.
to prevent this, every method in SQLiteDatabse should make sure
db is open before exec'ing sql statements. bug:2531172

Change-Id: I4cb5ab8539f46d7f8b26c3f830d799adf46444b6