Batterystats: fix inappropriate bg timebase reset

Previously, the background timebases (of a Uid) were reset when the Uid
resets in the wrong place. This caused StopwatchTimer.reset() to have the timesbase's old value to keep
track of its mUpdateTime. The solution is to call TimeBase.init at the
start of Uid.reset(), instead of calling TimeBase.reset() at the end of
Uid.reset().

Bug: 62352334
Test: runtest -x frameworks/base/core/tests/coretests/src/com/android/internal/os/BatteryStatsTests.java
Change-Id: I23c886544e18f154fc226cc81c22c3ea70fb4c7e