Add more first-party Brillo system users.

Brillo is converging on a set of mandatory system services to be
included in all Brillo builds. We'd like these services to run as
separate users just like other native Android/Brillo system services.

Add users for these services:
-Firewalld: Brillo has more granular firewall requirements (think e.g.
  Brillo-powered router.) Firewalld exposes a (currently) D-Bus/
  (soon to be) Binder API to broker access to IPTables rules.
-Trunksd: Some Brillo product configs will require TPMs. Trunksd
  manages TPMs in Chrome OS and Brillo.

Bug: 27299198

Change-Id: Ibe3303acdd9398ea52da0a37dbef73e5c5a948f7