Fix counting problems in StopwatchTimer.

Changed StopwatchTimer so that its count only increases if the timer is
started when its time base is running. Previously, if the time base was
off, the timer was started, the time base was turned on, and then the
timer was stopped, the count would be increased; now, it will not
(because the time base was off when the timer started). Moreover, this
likely fixes the count==-1 bug that previously could occur, since the
count will no longer be decremented if the timer is stopped after a reset.

Fixes: 36730213
Bug: 30099724
Test: runtest -x
frameworks/base/core/tests/coretests/src/com/android/internal/os/BatteryStatsTests.java

Change-Id: Iad195e431618629ce432074e0c1bd217f9818cb1