Another way to keep track number of foreground services in the device.

Add staic field sNumForegroundServices to keep track of number of
foreground services and apps in the device. This field is used
to report statsd atom LmkKillOccurred which can not hold
AMS lock.

Bug: 234618899, 243768727
Test: statsd_testdrive 51
Change-Id: Id4e53ecf8902c8373e79537a2732586c8cb553d5
Merged-In: Id4e53ecf8902c8373e79537a2732586c8cb553d5
(cherry picked from commit b860752a)