rusty-gd: simplify queued & pending commands

since packets are just wrappers around Arcs, copies are somewhat cheap

so no need to keep a struct with just opcode anymore

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost SimpleHalTest
Change-Id: I6294a728e07b2ee949ef96465b07678845f5115c