Add Crystalball Perfetto v2 metric spec for Ui Tracing slice durations

This is used to keep track of the average time of various Ui Tracing trace slices to monitor and track the performance of Ui Tracing better and consistently with the data we have in APC

Bug: 406267473
Flag: TEST_ONLY
Test: ./out/linux_clang_release/trace_processor_shell --summary --summary-metrics-v2 avg_runtime_per_uitracing_slice --summary-spec /usr/local/google/home/pablogamito/Android/main/frameworks/base/apct-tests/perftests/tracing/metric_specs/avg_runtime_per_uitracing_slice.textproto ~/Downloads/latest.perfetto-trace
Change-Id: I8b87a9b2684332196b9275449566ef6d427e9dcf