Set ril.radio.disabled False if True

@Askeing and Yoshi proposed that switching ril too often could be detrimental.

Sorry about the double negative but this pull re-enables the ril.radio only if it is disabled. I've moved it higher in the order to give the ril more time to change its state (although perhaps no difference in timing in reality).

In `test_settings_airplane_mode.py` also re-enable ril in the tearDown.

I've run this 3 times on Jenkins ad-hoc and believe it to be low risk to the test coverage but it is not clear that this will actually fix the problem of https://bugzilla.mozilla.org/show_bug.cgi?id=853489 , we can give it a shot.