BACKPORT: mm: fix pageblock heuristic

The Android-tuned page block heuristic was accidentally reset in an AU
drop. Fix the heuristic to avoid unnecessary unmovable pageblock
migration over time.

bug 30643938
Bug: 63336523
(cherry-picked from commit 3e19bcf7d08713daaaba888b4d13502e06e38e96)
Change-Id: I59efcd3934f29982b1c9aeb7b0f18eb17e0934b3
Signed-off-by: John Dias <joaodias@google.com>