[zx] Move public zx headers to match naming convention

The zx library provides headers intended for general use, so the
include path should be spelled <lib/zx/foo.h> as per
https://fuchsia.googlesource.com/docs/+/master/development/languages/c-cpp/naming.md.
This moves the headers into the proper location and updates includes to
match.

Change-Id: I6fc9b2a491b5f8b0d270c4dfc77fa4be5739071b

[zx] Update callers to zx::time and zx::duration types

The zx_time_t and zx_duration_t variants will be removed eventually.

Change-Id: Icc54983edc154eee2d1258160f921c3afc212ae0

[ulib][dispatcher-pool] Remove dispatcher pool from audio namespace as
it will be used for video as well.

Change-Id: Ibfdf0711a1c291b8661ed1fb2d3ab8280b676c79

[ulib][dispatcher-pool] Build with zircon build system and fix header paths

Also fix error path that was broken due to missing brackets in "if" statement

Change-Id: Ib5816ef716acc2ac60a7f05760b20942b5998855

[dev][audio] Move //drivers/audio project to zircon

Change-Id: I5b60b0134a9d2d788922244b81401bc55d9837e8