Revert "Set correct focus activity when using FLAG_ACTIVITY_REORDER_TO_FRONT"

This reverts commit 6d42df6d.

The original commit addressed b/25487874 by changing the root
activity to match the top activity. This however affects the
backstack behavior as we return to home if our current activity is
the root activity
(see ActivityStack::adjustFocusedActivityStackLocked). Upon
removing this change, the original issue no longer reproduces.

Fixes: 25784754
Bug: 25487874
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test CtsServicesHostTestCases android.server.cts.ActivityManagerActivityVisibilityTests#testReorderToFrontBackstack
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test CtsServicesHostTestCases android.server.cts.ActivityManagerActivityVisibilityTests#testReorderToFrontChangingStack

Change-Id: I291fab0b14c69ec8c4c8ce3100cd391260a4c77e