Report task id for a finishing root activity

Task#getRootActivity() uses setToBottomIfNone=false which will
ignore all finishing activities. That causes Activity#isRootOfTask
to return a different result if the root activity is finishing.

So change to check ActivityRecord#isRootOfTask() that uses
ignoreRelinquishIdentity=false and setToBottomIfNone=true, which
can return the actual hierarchy root of task.

Fix: 420539568
Flag: EXEMPT bugfix
Test: TaskTests#testGetRootActivity_allFinishing
Change-Id: I659cdf4f433cee3dd9da2f6ce604eb29182dfe06