Ensure DeviceConfigProxyFake calls listeners correctly.

Prior to this change, all properties would be passed to
OnPropertiesChangedListeners that were registered on a
DeviceConfigProxyFake, regardless of whether they changed or not.

Fixes: 162744563
Test: atest SystemUITests
Change-Id: I2801e4fb828c6aec797d2a99f3f465a820664a53