Log dream_setting_changed to statsd

This change pushes a log to statsd whenever dream setting is updated.

Bug: 284029342
Test: Test: statsd_testdrive 705
Test: make -j64 RunSettingsLibRoboTests ROBOTEST_FILTER="com.android.settingslib.dream.DreamBackendTest"

Change-Id: I73d85daa0998d94c5892b7b8f5af6891383fcd3b
2 files changed