[PartitionAlloc] Re-enable the thread cache on low-end Android.

When thread cache purging is enabled, keep the thread cache on low-end
Android devices, but with reduced thresholds.

Bug: 1169159
Change-Id: I7288287a162d3500c3150a63f1bc5d95e61e5a51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2737116


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#860162}