BroadcastQueue: constants via DeviceConfig.

Move our initial set of configuration knobs to be loaded and updated
via DeviceConfig inside BroadcastConstants.

This leaves BroadcastConstants with a mix of queue-specific values
and global values; we leave TODOs to detangle this in the future.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: I367d23eedacf132759cc7e4b6dab0e07da6e2253