power: Add property CHARGE_COUNTER_EXT and 64-bit precision properties

Add POWER_SUPPLY_PROP_CHARGE_COUNTER_EXT that stores accumulated charge
in nAh units as a signed 64-bit value.

Add generic support for signed 64-bit property values.

Change-Id: I2bd34b1e95ffba24e7bfef81f398f22bd2aaf05e
Signed-off-by: Todd Poynor <toddpoynor@google.com>