Update registerOffloadEngine() permission check to check the
DEVICE_POWER permission in U. This change is required to allow the
android TV device to access the API in U because the
REGISTER_NSD_OFFLOAD_ENGINE permission can not be backported.

Bug: 313546516
Test: TH
Change-Id: I84b80d102a34487ad54719a86eb525b319e2fd8b