Increase testStopAllTasksForUser test case stability

1. Used ActivityManager.getRunningTasks to check if there are running
   tasks for a package instead of ActivityManager.getRunningAppProcesses
2. Still Ignore the test as it is not able to run 100-iteration
   reliably.

Bug: 232432706
Test: atest
CtsCarBuiltinApiTestCases: ActivityManagerHelperTest#testStopAllTasksForUser
Change-Id: I19f21a9ed2b0d3f2ac49d87ebb5d2faf05d07f04