pixelstats: MM Metric: fix compaction bad write

* Fix a known bug for compaction duration metrics
    - incorrectly writes to some internal data structure belonging to
      another entity (i.e. data used for direct reclaim metrics)

Bug: 334554581
Test: atest pixelstats_mm_test
      Note: The test results show fewer mismatched data, and confirm
      the fix.
Change-Id: I9e5dc1c3c4900a18c9847c1c636d24dbf4fe1458
Signed-off-by: Robin Hsu <robinhsu@google.com>