BatteryStats: Introduce Async external stats requests

Instead of calling out to external processes with a blocking IPC,
pass along a Binder on which the external process can pass back
the response. The calling process can then wait for the reply with
a timeout.

This eliminates watchdog restarts of the system_server when an external
process like telephony or bluetooth hangs.

Bug:26842468
Change-Id: I1b242e4ed22a63f1a4a0be8c78de8ac4d7bf56c5