rusty-gd: back message loop thread with rust, if rust enabled

the rust code runs in a single threaded environment.
since the main thread is now implicitly running in this environment too,
this means we no longer need to transfer contexts in the shim

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run
Change-Id: I05ec50c525184c29627e1eaff2b62bac0b879416