dumpstate: attach more than 1 trace to BR

perfetto now allows stopping multiple ongoing traces by using
`--save-all-for-bugreport`. This change serializes all the
traces, not just the main one, in the BR.

Bug: 321196572
Flag: perfetto.flags.save_all_traces_in_bugreport
Test: atest dumpstate_smoke_test  -- --test-arg com.android.tradefed.testtype.GTest:native-test-flag:"--gtest_filter=DumpstateTracingTest*"

Change-Id: Ibf6e2c7d9620f49cd50a49716586b9b29c94e6a7