Fix PowerKeyGestureTests#testPowerSinglePress

In ag/23249820, we moved the code to fetch power key behaviors from
PhoneWindowManager#init to PhoneWindowManager#updateSettings. Since the
PowerKeyGestureTests mocks out the updateSettings method, the key
behaviors were not being read properly.

This CL fixes the issue by explicitly setting the power button short
press behavior (which used to be implicitly assumed in the specific test
case).

Bug: 287561423
Test: atest com.android.server.policy.PowerKeyGestureTests#testPowerSinglePress
Change-Id: I446efafb5695d8fc76aa75d2338d8862e538cb01