[native] Make SecureStore methods static

Summary:
ENG-1567

This will make it easier to expose SecureStore to Rust as our native Rust codebase as inherently stateless.

Test Plan:
Tested on Android and iOS physical devices:

• Add log to SQLiteQueryExecutor::initilize that prints maybeEncryptionKey read from Secure Store
• Open the app -> encryption key is logged
• Log out and log in to trigger creation of a new encryption key
• Restart the app -> a different encryption key is logged

Tested notifications on physical devices:

• Send notification - ensure it is displayed correctly.
• Send notification to backgrounded app, kill keyserver in the terminal and open the app. Ensure that data delivered with notification are present even though the keyserver is not running.

Reviewers: marcin, kamil

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9923