Add method to set entire sttring set at once in MMKV

Summary: This differential adds method that allows to set entire set of strings at once in MMKV. It will be necessary since push-handler component wil update set of unread thread IDs in MMKV when it is connected to Tunnelbroker

Test Plan: tested in the diff after the next one.

Reviewers: bartek, kamil, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13188