Fix crash when rotating HighPowerDetail dialog

When rotating the device, the Activity of the target
Fragment become null. So NullPointerException occurs.
The dialog should check if the Activity is null
before calling onActivityResult().

Bug: 28333723
Test: manual - go to Settings > Apps > Clock,
               click Battery optimization and rotate device

Change-Id: I7f68d6ece7d169fd96d04c1788e55ee506aae1ae