Initialize the Timer::splits pointer

This addresses a review comment: the Timer::splits field was not
initialized in the default constructor for Timer.  It is not
initialized to the null pointer.

Flag: EXEMPT bug-fix
Bug: 407584606
Test: atest
 * FrameworksServicesTests:AnrTimerTest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksMockingServicesTests:com.android.server.am
Change-Id: I0ab74209b702d270db80beb59056d64669c20530