Properly set isInstantApp for power usage page

Previously the app header was not properly configured to be aware
it was rendering an instant app. This change properly sets the value
and adds tests to ensure it is correclty set in the future.

Test: make RunSettingsRoboTests -j30
Bug: 37919863
Change-Id: I0cbe2cb03bcd44208408f68188387e53ae98d8fb