[1/?] Reduce BroadcastQueue interface complexity.

Start pruning down the methods surfaced through BroadcastQueue,
which reduces the surface area future implementations will have to
think about.

Begin adding documentation to well-understood methods.

Bug: 243656033
Test: atest CtsContentTestCases:BroadcastReceiverTest
Change-Id: I401b61640b38a677e1bf9b8986475daa044122e0