Fix testQuickSwitchFromApp

Instead of starting getAppPackageName() and relying on it being our Test
Pin Item activity, instead launch our own test activities with the
FLAG_ACTIVITY_MULTIPLE_TASK and FLAG_ACTIVITY_NEW_DOCUMENT flags.

Test:
- Locally run testQuickSwitchFromApp() from Android Studio
- flake -oop -t com.android.quickstep.TaplTestsQuickstep#testQuickSwitchFromApp

Bug: 140252765
Change-Id: Ie137261ce65bfd3dd39df78d57784854a026e967
Merged-In: Ie137261ce65bfd3dd39df78d57784854a026e967