Fix DozeConfigurationTest

Settings.Secure.putString() was failing to remove the setting for
config_dozeAlwaysOnEnabled, so the test would only pass if the default
setting matched what was currently set on the device.

The fix seems to be to use
Settings.Secure.putStringForUser(UserHandle.USER_CURRENT), because
systemui tests are now running as the system

Test: atest com.android.systemui.doze.DozeConfigurationTest#alwaysOn_followsConfigByDefault
Change-Id: I482e1b72b4a3567d5448ded0aa110f2e3e05c515