Turn off prefetch constraint relaxation on battery.

Only let prefetch jobs use the opportunistic quota when the device
is on charger to reduce excess battery drain. The opportunistic quota
usage behavior is optional and not required for prefetch jobs, so we can
disable it without impacting app or developer expectations.

Bug: 295046507
Bug: 299329948
Test: atest FrameworksMockingServicesTests:ConnectivityControllerTest
Test: atest FrameworksMockingServicesTests:PrefetchControllerTest
Change-Id: Idc137279fea1100f53ecf20279153fe92fdb5617