aconfig: introduce namespace, rename proto messages

Flags belong to a namespace. Update the proto files to reflect this.

Config files can only refer to a single namespace. Override files can
refer to multiple namespaces; an override directive for a flag in a
different namespace than the one represented by the cache will be
silently ignored.

Rename the proto messages to make it more clear what they are. Propagate
this change through the wrappers in aconfig.rs and the rest of the code.

Also, settle on Tracepoint instead of TracePoint.

Bug: 279485059
Test: atest aconfig.test
Change-Id: I16e69dd14687bc498b2ba89d6a35879459903801