statsd: Add death recipient to PowerStatsPuller

PowerStatsPuller now will correctly re-register with the PowerStats HAL
if the PowerStats HAL service is killed.

Bug: 120919173
Test: adb shell cmd stats pull-source 10038
Test: Killed PowerStats HAL service, waited for it to start back up,
and verified that statsd was still able to pull the PowerStats.

Change-Id: Iddaab55d70e785785402d27ad0abca9911832747