[SES-3593] - Disable KDF for sqlcipher

[SessionHero01]

This PR effectively disables the kdf in sqlcipher. The reason we want to do this:

1. The key used in sqlcipher is a securely generated key rather than a passphrase. There is no need for further key derivation.
2. The sqlcipher's builtin kdf adds significant time on start up: as we spawn many db connection on app start-up, and each connection needs to go for 256000 iteration of kdf and it adds significant time on cold start. I believe this is where most of our ANRs come from.

For reference document on kdf, see https://www.zetetic.net/sqlcipher/sqlcipher-api/#kdf_iter

This PR also moves the initilisation of SQLCipherOpenHelper to a lazy loading fashion so that the expensive migration will only happen when database access is required. This means all the database classes shouldn't access to the helper directly, they should all use the getReadableDatabase/getWritableDatabase from the base class.