Use baseIntent to obtain package name

baseIntent's component is more often non-null than baseActivity's, so
use that when obtaining the number of other instances.

Flag: com.android.window.flags.enable_desktop_windowing_multi_instance_features
Fix: 387552315
Test: atest WMShellUnitTests

Change-Id: I1b01b1ec8b265b9df0dd1435a4656cf3b7f72ba0