iOS: change magnetometer implementation to use Timer base polling

Ref change: 102bdf3

We need to change the implementation to use polling rather
than callback to achieve full performance together with
fine-grained QTimers.

Change-Id: Ic3bf978633c56175eeca90cad8ed764a0b1c9b0c
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>