iOS: added helper functions to gate image instrumentations

Summary:
Introduced 2 helper functions to toggle image instrumentation/logging (not in this diff) so that gating check is more efficient and easy to access across RN iOS core.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D18597208

fbshipit-source-id: 0b22031802ab020b16d6fb63e52461cf80a37ab5