power: pm8921-charger: BTC override fixes

- Monitor battery health when charger is not connected
- Add configurable temperature hysteresis and enable the
  same for BTC override
- Avoid releasing wakelock when charging is not in progress
  and BTC override is enabled
- check temperature thresholds when fully charged, to handle
  temperature restrictions while restarting charging

CRs-Fixed: 466525
Change-Id: I8cad3f8bcf65844fc1048625d9b387ede132d3e0
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>