Persist TaskRecord before calling performClearTaskLocked.

A recent change persisted this record with a member variable that
could be cleared as a result of performClearTasklocked. This
changelist resolves this by using a local variable to store the
reference until after the method call.

Change-Id: I6d5d42e06afdbf6e8ff960fcfae71c80abd891f4
Fixes: 36001400
Test: adb shell am instrument -w -e class com.google.android.setupwizard.tests.activity.SetupWizardExitActivityTest com.google.android.setupwizard.tests.activity com.google.android.setupwizard.tests.activity/.SetupWizardTestRunner