[sensorfw] need to run init after sensord has been restarted

This ensures things are setup and work properly.

Change-Id: I23c8b53e760d49695c5ae207eed77a2e7d2be3a4
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Series of crash fixes from Mer, squashed

        1)tracing plugin loading

        2)m_remoteSensorManager can be nullptr

        But it seems it does not solve incorrect initialization in the case
        manager interface is nullptr: init() for different sensors does not
        check initDone

        3)split initSensor() to template function and function

        most of initSensor() does not depend on template param, so it is
        better to split it -> less code should be generated

        4)do not use sensor manager if it is invalid

        5)correct class members initialization order

        6)assert sensor interface is not null

        From what I saw according to logic it should not happen so using
        Q_ASSERT for better maintainability

        7)check local interface pointer in SensorfwTapSensor::start()

        Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>

Change-Id: I5c1bf3999ad2268c0dba9b3fe511d999c2e63fd9
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>