[CDM] Refactor CDM association discovery timeout mechanism.

* Spinner was changed to an indeterminate progress bar to indicate ongoing scan.
* Discovery timeout no longer results in automatic cancellation.
* Single-device association flow now shows a system dialog.

Bug: 336830254
Flag: EXEMPT bugfix
Test: Manually tested on a test app
Change-Id: I247d30c8fc756a0477974e773b7933ff5cd7c8c6