fix(liveness): Prevent the "Create Liveness Session" button from flashing on screen transition

[vincetran]

• PR conforms to Pull Request guidelines.

Issue #, if available:
N/A

Description of changes:
Because the fetchingSession.value gets set to false on session retrieval, it has a side effect of showing the "Create Liveness Session" button for a split second before the screen transitions to the LivenessScreen. By only resetting it on exception, we can prevent that flashing from occurring.

How did you test these changes?
Manually, the button no longer flashes and no regression on exception.

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(liveness): message, fix(authenticator): message, fix(all): message)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.