Require an app activity for launching UIs.

Apis (get/create credentials) that involves UIs now requires an app
activity as an input paramter that's used to launch UIs throughout the
flows. This way the UIs will be launched within the app activity task
stack.

Bug: 246564035
Bug: 253156924
Bug: 253156958
CTS-Coverage-Bug: 246637346
Test: Local Build & Deployment
Change-Id: Ia099c387adce516ebc52deb9ae00a6b046e9d296