Enable ShadeViewDiffer systrace logs

 - Enable ShadeViewDiffer "UI Events" systrace logs on debug builds

 - Guard against unnecessary string creation in
   LogBuffer.echoToSystrace() when tracing is disabled

 - Add two new trace sections for debugging ShadeViewDiffer.applySpec()

Test: Capture trace, look for UI Events during applySpec
Bug: 224973321
Change-Id: I438947f43a9fc1bc6ff66e923645fd695cc99543