shell tests: add EnsureDeviceSettingsRule

Mainly to make sure that Settings.Global.STAY_ON_WHILE_PLUGGED_IN
is enabled to avoid turning off the screen and increasing jank
metrics during long tests.

Flag: EXEMPT minor change
Bug: 323165543
Test: presubmit
Change-Id: I22df997d9657400a268818146e53b0d81e9127fa