traced: bail out if an invalid trigger mode is specified

This is a fix to a subtle binary back-compat issue.
When introducing a new trigger mode, the service will ignore it
and trace indefinitely (Because duration_ms is checked to be 0).
The issue is not as bad as initially though because perfetto_cmd
mitigates this, by enforcing a hard timeout to 60s + trigger_timeout
regardless of the type of the trigger (See comments in b/274931668).

Bug: 274931668
Bug: 260112703
Test: perfetto_unittests --gtest_filter=TracingServiceImplTest.FailOnUnknownTriggerMode
(cherry picked from commit 7f432673ea568e8ab9b7f4ccfe0107a4c5e01e81)
Change-Id: I7993ec7a3604b8ede00b3c29cf29059685af4848
Merged-In: I7993ec7a3604b8ede00b3c29cf29059685af4848