gd: Alarm should take const ref, not mut

Alarm's reset and cancel methods don't require mut references. Making
them const ref allows us to create and use `Arc<Alarm>`, which is safe
because AsyncFd implements Send + Sync.

Bug: 193261630
Test: atest --host bluetooth_test_common
Tag: #gd-refactor
Change-Id: Ia057815593abb1fed504c5c9d9dce7ce8b961151