project: host_commands: Add type annotations to appease mypy

Some imports cannot be resolved because the linter cannot locate them.
Also remove unused field `Microdroid.stop_thread`.

Bug: None
Test: mypy --check-untyped-defs project/host_commands/microdroid-test.py
Change-Id: Ifdb6c593eb73f29b3f5c8c58ed1160c309648d28