KernelWakelockReader: provide class level lock when updates staleStats

As the wakelock version number is defined as static to provide
consistent versioning across objects, class level static lock should be
implemented to prevent racing conditions.

To trigger the racing condition, update statsd's stats pulling logic
locally to repeatably requesting wakelock stats then wakeup the phone
to trigger BatteryStats update routine. The racing condition is 100%
reproducible under the setup. The patch has been verified with the
setup, and the racing is no longer seen. See more reproduce details in
the linked bug.

Bug: 173539101
Test: manual
Change-Id: I386afa2f2ecd8678e71ece978da4a9950b21ca4d