Alarm: Create timerfd with TFD_NONBLOCK flag

In order to prevent the thread from being blocked by blocking read()
in alarm's on_fire(), this CL creates the timerfd with non-blocking flag.

Bug: 344712454
Bug: 360743527
Test: atest bluetooth_test_gd_unit
Test: atest bluetooth_test_with_timerfd
Change-Id: I23b8e633abd4a46131bbe98c3f2866dd96d5142e