power: qpnp-fg: qualify capacity learning start conditions

Add an additional check to the capacity learning algorithm's start
conditions to make sure that the battery is reasonably well settled
before starting. Specifically, check that the difference between
vbat_shadow and the v_current_predicted is not greater than a specified
threshold before starting the algorithm.

CRs-Fixed: 827068
Change-Id: Ief96061c69d26c8ebbf4cf374c3e0f7ef5933017
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>