Fix the unit tests for WebView downgrade logic

Shift the branch numbers by 5 digits to construct the versionCode according to
Chromium versionCode scheme.

Changed "non-system version" to "candidate version" because a package under
comparison could be a system package too.

Also add a new test for the fallback being used case.

BUG: 28050853
Change-Id: I25becb713a8840e579ded55b8882224c6b69e1bb