[PA, MTE] Fix PartitionAllocTest.FewerMemoryRegions and InaccessibleRegionAfterSlotSpans' failures on android-mte bot.

Sample failure:
https://ci.chromium.org/ui/p/chromium/builders/ci/android-mte-arm64-rel/2207/overview

To get `address` from `pointer`, we need to use `UntagPtr(pointer)`.

Cq-Include-Trybots: luci.chromium.try:android-mte-arm64-rel
Change-Id: I0ef260236274def1a5fc1e84beece6d4e5ad3257
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6180108
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1409486}
NOKEYCHECK=True
GitOrigin-RevId: a581171345614759be624b129d2347691c9823f3