[ADI][unit test] Use `mUserId` instead of hardcoded 0 in `DeveloperVerifierControllerTest`.

The test calls to `startVerificationSession` now pass the `mUserId` member variable instead of a literal `0`, improving test clarity and correctness especially on Auto builds.

BUG: 441284150
FIXES: 441284150
FLAG: android.content.pm.verification_service
Test: presubmit
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:2bf5057878f1d1497ff0edf9e952d0bab4bee724
Merged-In: If97da86fae10075c5a3ea60413e630faea078cdb
Change-Id: If97da86fae10075c5a3ea60413e630faea078cdb