Fix double-detach DualTimer bug

DualTimer attempted to detach its subTimer twice when reset(true) was
called, once explicitly and once via a call to the main timer. This
fixes that problem by getting rid of the explicit detach in reset.

Bug: 37208694
Test: runtest -x
frameworks/base/core/tests/coretests/src/com/android/internal/os/BatteryStatsTests.java
and manually looked for "Removed unknown observer" error in logcat.

Change-Id: Ic5ff7d799d46236a74ab0825e108bef40bac0360