ANDROID: gki_defconfig: reduce KFENCE pool size

CONFIG_KFENCE_NUM_OBJECTS controls the constant memory overhead that
KFENCE introduces for its memory pool. By default it is 255 objects
(2Mb extra memory), but since concerns have been raised that low-memory
devices may not afford that, we are lowering the number of objects
to 63 (512Kb extra memory).

So far we haven't seen Android devices allocate more than 50 KFENCE
objects. Should the kernel exhaust the pool, KFENCE will stop allocating
new objects and fall back to SLAB/SLUB until one of the objects is
freed.

An immediate consequence of reducing the pool size is that a freed
KFENCE object will be reused 4x times faster, effectively reducing the
probability of detecting a use-after-free. Since KFENCE is a best-effort
error detection tool, not a use-after-free mitigation mechanism, we
believe this should not be problematic.

Bug: 181822795
Test: run KFENCE on an Android device
Signed-off-by: Alexander Potapenko <glider@google.com>
Change-Id: I88fca80eea8b382115de03dbb104fd305b35476f