Fix failure of ZenModeAutomaticRulesPreferenceControllerTest

Running ZenModeAutomaticRulesPreferenceControllerTest fails due to
missing field "DEFAULT_RULE_IDS".

"mDefaultRuleIds" should be used instead of it.

Bug: 63077372
Test: make \
    ROBOTEST_FILTER=ZenModeAutomaticRulesPreferenceControllerTest \
    RunSettingsRoboTests

Change-Id: Ieb4a4304a0404685e4a50eb3a5c863a0132ebf7d