Delay copy-buffer allotment for uffd compaction until actual use

Currently, we allot copy-buffer to mutator threads as soon as a
userfault for moving space is received. This could lead to a scenario
wherein a lot of mutator threads try to access the same page
simultaneously and hence get userfaulted. In this case we would
uselessly allot buffers to all mutator threads.

In this CL we defer the allotment until it is confirmed that the
mutator thread needs the buffer for compaction.

Bug: 284289925
Test: art/test/testrunner/testrunner.py
(cherry picked from commit 33e2f0e1caecab9c7b681e841e500e87caaa540f)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:684b6c42ab68ed1c10616ea076c23fb22a5ff354)
Merged-In: I9201716227e9e7245f20b016ff86558e0842fc53
Change-Id: I9201716227e9e7245f20b016ff86558e0842fc53