revert parts of dc5b63e4, which made gyro drift estimation unstable

    initialize the system covariance matrix with non-zero
    values for the gyro-bias part. this improves the initial
    bias estimation speed significantly.

the initial covariance matrix should be small because the drift
changes slowly. the real problem is that we're not starting with
a good estimate of the drift, which this algorithm relies on.

so with this revert, it'll take a while for the drift to be estimated
but it won't be unstable.

Change-Id: Id5584bc114a2390d507643b2451b2650c1b90721