[native] introduce atomic flag that indicates clearing database process will execute

Summary:
context: ENG-2139

The flag will allow indicating (while executing task) that the task should be canceled (because e.g. clearing the database was scheduled).

Test Plan:

1. Build android app
2. Build iOS app
3. check on logout (by adding logs) that the flag was properly set and unset after calling cleatSensitiveData.

Reviewers: marcin, jon, tomek, atul, ashoat

Reviewed By: marcin, tomek, atul, ashoat

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5576