Fake timer periodicity should be set based on it_interval field of

itimerspec

Currently, we determine fake timer period based on the it_value field of
itimerspec. This is incorrect. Instead we should use the it_interval
field of itimerspec.

Bug: 244313799
Tag: #gd-refactor
Test: gd/fuzz/run --host hci_layer

Change-Id: I825d56c8d5eb7e3d22f9a4006b6cf7f737ffcd63