examples/radio-observer: Remove duplicate timer initialization
ll: Modify interval parameter to uint32_t
bci: Modify interval parameter to uint32_t
timer: Change timer_start() API from ms to us

In several places we need to have timers with microsecond resolution.
This patch modifies the timer_start() API to accept us, adds helper
macros TIMER_MILLIS() and TIMER_SECONDS(), and updates all examples.
ll: Enforce advertise interval to be multiple of 0.625 ms