Add dream complications enum and Settings API.

In order for users to toggle complications on/off in Settings, we need
to categorize complications by type and store user preferences in
Settings.

Bug: 214038424
Bug: 214039192
Bug: 214038263
Test: make -j64 RunSettingsLibRoboTests ROBOTEST_FILTER="com.android.settingslib.dream.DreamBackendTest"
Change-Id: I2db4f2f9c1e08c6cfac3687fa3809662f30a3561