BatteryStats: Increase max size of subsystem state string

New devices may have many more subsystems, which was resulting in
missing data in batterystats history.

Bug: 168156597
Test: Run device and trigger batterystats history events. Verify that we
don't hit the max buffer size indicated by a log message.

Change-Id: I2f23dd1dbb4df5c3d9680d7087967563acb72197