sdk: Add track_event_buf_exhausted_policy to TracingInitArgs (#5409)

Add a track_event_buffer_exhausted_policy field to TracingInitArgs,
allowing apps to set the buffer exhausted policy (drop vs stall)
for TrackEvent at initialization time.

The value is stashed in TrackEventInternal static state during
Tracing::Initialize() and read during TrackEvent::Register(),
following the same pattern as use_monotonic_clock and
disallow_merging_with_system_tracks. A protected Register(dsd,
params) overload on DataSource allows TrackEventDataSource to
inject custom DataSourceParams without intermediate storage.

TrackEvent is used in critical system services (e.g.,
system_server) where an accidental kStall policy could cause a
device soft-reboot. Plumbing through TracingInitArgs removes
ordering ambiguity (Initialize always precedes Register) and
avoids redundant state in DataSourceType. See
https://github.com/google/perfetto/pull/1312 and
b/384007571.

I don't have a use-case for also allowing track_event_buf_exhaust_policy
to be configurable.
For now, making a hard-choice when we initialize the process is ok.
If we need configurabilty, the extension points are natural; we can
follow footsteps of this PR.

cc: @dreveman