Correct null check for end button behavior.

SettingsState.getSettingLocked never returns a null value, so the car_product overlay value of this setting is always ignored. PhoneWindowManager then uses a hardcoded default with undesired behavior for the car. Checking the null indicator on the returned value allows the setting to be initialized properly.

Fixes: 113123033

Test: adb shell input keyevent 6 before and after change, verify desired behavior
Change-Id: Ib2b64331ae986efda6bb961d487e4de023c43d84