Add charger related property contexts

ro.enable_boot_charger_mode and sys.boot_from_charger_mode are moved to
new property contexts for charger props to remove exported*_prop.

Bug: 155844385
Test: boot device with ro.enable_boot_charger_mode
Change-Id: I17d195d3c9c002a42125d46a5efcdb890f1c2a5c
5 files changed