Fix for "null_(vnull):[formID]" schema names

Signed-off-by: Matthias Stevens <matthias.stevens@gmail.com>
Progress on SQLite back-end (nearing completion):
- moved Android-specific classes to CollectorAndroid
- new cleanup() method (called upon finalise()) on RecordStore
- in SQLRecordStore#cleanup() looks for schema tables that have become empty and drops them, along with deleting the corresponding row from the Schemata table.
- SQLiteRecordStore.SQLiteTable now overrides drop() in order to dispose any open statements/cursors before the DROP operation (otherwise the later fails on SQLite4Java due to a "database table is locked" error)
- got rid of NullRecordQuery (it wasn't used anymore)
- doBackup(File) in JavaSQLiteRecordStore now uses the SQLite Online Backup API which is exposed by SQLite4Java (instead of doing a simple file copy as in SQLiteRecordStore#doBackup(File))
- new isRecordInDB() method in SQLRecordStore.SQLTable and overridden in SQLiteRecordStore.SQLiteTable to allow for a more efficient way of answering the question of whether a record is new (i.e. using a cached prepare query)
- etc.

Signed-off-by: Matthias Stevens <matthias.stevens@gmail.com>