Use atomic++ rather than lock/++/unlock in the input dispatch code path

Decouples the input dispatch thread from the battery-stats object lock regime,
to avoid the possibility of ever blocking the input dispatch thread on its
behalf.  The stats object is widely used and can sometimes be locked for a
very long time (on the order of seconds) during certain extensive dump
operations.

This change does not alter the data format of the battery stats' externalized
representations.

Fixes bug #2530346

Change-Id: Iee288be3bf4936641b532dceecb8f6de8f552bf0