perfetto: Add opt-in shift-left compaction to TraceBufferV2

In long streaming sessions the consumer periodically drains the buffer,
but the freed space stays resident: consumed chunks are only turned
into padding, so a buffer keeps roughly its full size in dirty pages
even when almost everything in it has been read.

Teach TraceBufferV2 to compact itself in BeginRead(), the only point
where no read iterators or packet slices can be outstanding. When at
least 64 KB is reclaimable and at most 1/8th of the initialized region
is still live, live chunks are packed to the front of the buffer
preserving ring order, the per-sequence chunk index is re-pointed to
the new offsets, used_size_ shrinks accordingly and the now-unused
whole pages are released back to the OS via AdviseDontNeed().

The feature only applies to kOverwrite buffers and is gated by the
tbv2_shift_left_compaction aconfig flag (default off). A new
per-buffer `compactions` counter in TraceStats.BufferStats makes the
behavior observable in traces.

Bug: 536851377