Finally fix TestModeBuilder so that it clones DND correctly

Specifically, new TestModeBuiler(dnd).build() will now return a proper DND mode, instead of a half-DND-half-rule hybrid.

Removed workaround in DoNotDisturbQuickAffordanceConfigTest and ClockEventControllerTest to verify it works. Other existing tests still pass.

Fixes: 381413390
Test: atest DoNotDisturbQuickAffordanceConfigTest com.android.settings.notification.modes
Flag: TEST_ONLY
Change-Id: I666e27651b89ce3c2caf190dce6ff2cc8d9b2eda