healthd: Remove libhealthd dependency from charger

Clean up charger's libhealthd dependency.

- Charger uses libhealthloop to maintain an infinite
  loop, similar to all health 2.x services.
- Charger tries to open up health 2.1 HAL implementation
  to retrieve health info. If it failed, it falls back
  to the legacy code path where a default BatteryMonitor
  is used, *except* that it won't depend on libhealthd's
  healthd_board_init() and healthd_board_battery_update()
  anymore.
- Remove global static variables because they are hard to
  track.
- Modernize code by converting charger_state in to a C++
  Charger class, transforming all functions into methods,
  and moving all other global states into the class.

- Devices that matches all of the following:
  - have a customized libhealthd (search for modules
    named libhealthd.xxxx)
  - uses charger from system image (look for "class charger"
    in device init.rc scripts; if you see the binary is named
    "/charger" or "/system/bin/charger" then you are using
    charger from system image)
  ... must implement health 2.1 passthrough implementation
  properly in order to have charger continue to work.

See hardware/interfaces/health/2.1/README.md for details.

Test: charger test
Test: manual charger mode
Bug: 127677771
Bug: 142286265

Change-Id: I0f26e5c1fe2be6b5952fc019224457c8419e43e4