TrackEvent: Make process/thread naming less of a footgun

When calling TrackEvent::Set{Process,Thread}Descriptor, it's tempting to
set the |name| field of the returned TrackDescriptor to record the name
of the thread or process, but this only ends up setting the name of the
track object which is associated with the thread or process, and is not
directly displayed by the UI.

Instead, one should name the thread or process by filling in the
{Process,Thread}Descriptor message inside the TrackDescriptor. This
patch makes the API more conducive to doing that by accepting
protos::gen track descriptors -- which are more easily mutated by the
user -- while maintaining backwards compatibility with the original
behavior.

Bug: 155076522
Change-Id: I7c8c9032c8e7d59a28c90c81a2b11424a7bce5e2