Make SDK sandbox BAL check multi-user aware

The existing logic does not work for multi-user apps, as the
BAL logic checks whether the user 0 app uid has an active
visible window. Instead, use the multi-user uid for this check.

Test: Manual
Test: atest ActivityStarterTests
Bug: 287476371
Change-Id: I2e2bffb4c38ad43d50f48fa71345d9b7bb605272
Merged-In: I2e2bffb4c38ad43d50f48fa71345d9b7bb605272