A lot of modem code requires sleeping while vendor services do some
background processing. Since we don't want to actually sleep for unit
tests, an interface is provided here so that a fake sleep can be
injected.
Test: N/A. Directly forwards methods or does nothing.
Bug: 302435001
Change-Id: I3bcf0307156d93756d69cd9f749c88b508ba9466
0944a8db52