Systrace fixups and refactoring for TimingLogger

Fixed systrace bounds for timing logger in GC. Refactored CumulativeLogger a bit to
allow for multiple identical entries. Added ScopedSplit, now composes with explicit
start/end/new splits. Adds some unit tests.
Bug: 10036801

Change-Id: If0afb88b48ec3a1e19462ed354babb274a9517a7
7 files changed