charger: separate UI and HAL access.

Abstract all the HIDL-ness from charger so an AIDL HAL
can later replace the HIDL HAL.

Created libhealthd_charger_ui that is responsible for
UI stuff in charger. Then libhealthd_charger is now the
charger library backed by HIDL HAL.

Bug: 170338625
Bug: 203246116

Test: charger_test

Change-Id: I193bb76afa18b3367f24796ac53b1760650e8683