Introduce MMKV to Android app

Summary:
This differential introduces MMKV to Android app. For now only string getter and setter is implemented but future diffs will introduce new methods. This
differential introduces simplest API we can test.

Test Plan:

1. Below the CommMMKV.initialize() line add line that sets value under a string key.
2. In CommNotificationsHandler.java at the beginning add line that tries to get string from the same key and logs this key in case it is present.
3. Send notification and check it the same string is logged.

Reviewers: tomek, bartek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11046