Separate out fake timer.

We need fake timers for unit tests. Fuzz tests also use fake timers.
This CL separates out fake timers from fuzz testing code so that they
can be used independently.

Bug: 243662087
Tag: #gd-refactor
Test: gd/fuzz/run --host acl_manager

Change-Id: I83b0d80ee71df24c435b411cab3ff7375fe4ba86