isFeatureEnabled now returns false if disabled

[marandaneto]

What does this PR do?

isFeatureEnabled should return false if feature flag is disabled

Where should the reviewer start? PHGPostHog.m

How should this be manually tested? Demo project has one flag for each case, you can just call isFeatureEnabled with them.

Any background context you want to provide?

Relates to https://github.com/PostHog/posthog-android/pull/42

What are the relevant tickets?

Screenshots or screencasts (if UI/UX change)

Questions:

• Does the docs need an update?
• Are there any security concerns?
• Do we need to update engineering / success?