Move database encryption utils to generic location

Summary:
AES_GCM functions used to encrypt/decrypt database are very generic and can be directly applied to olm notifications session encryption before it is
persisted in IndexedDB. We only need to move them to generic location and rename them.

Test Plan: Run yarn jest all since it wil run worker-crypto-utils.test.js that use all moved functions. This ensures that existing functionality wasn't broken.

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9619