Make class SetTimerSlackAction easier to maintain

Unify the Darwin and Linux class declarations. Move all #ifdefs for this
class into the .cpp file. This patch does not change any functionality.

Bug: 213617178
Test: Booted Android in Cuttlefish.
Change-Id: I6a3cedc75c2dbf8aa062edda76cae9959767a763
Signed-off-by: Bart Van Assche <bvanassche@google.com>