healthd: charger: shutdown timer configurable

config the shutdown timer via the property system
for vendor customize.

Bug: 129599884
Test: setprop ro.product.charger.unplugged_shutdown_time 5000
      getprop ro.product.charger.unplugged_shutdown_time
      5000
      dmesg:
          charger: [371812] device unplugged:
                   shutting down in [5000] (@ 421812)

Change-Id: Ifa377cdb8e100c453346522a3eb93379fd7e2567
Signed-off-by: John Zhao <yuankuiz@qualcomm.corp-partner.google.com>