[Android] Promote folly to library target

Summary:
Other CMake modules in native/cpp also need folly, so
we need to have folly as a well defined target which those files
can consume.

https://linear.app/comm/issue/ENG-2895

Test Plan: Android gate passes

Reviewers: max, bartek, varun, atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6484