[PA] Refactor and merge PA/AC shim logic

This patch merges the PartitionAlloc with Advanced Checks shim
delegation logic directly into the main PartitionAlloc default dispatch
file.

Changes:
- Merged allocator_shim_default_dispatch_to_partition_alloc_with_advanced_checks.{cc,h}
  into allocator_shim_default_dispatch_to_partition_alloc.{cc,h}.
- Renamed InstallCustomDispatchForPartitionAllocWithAdvancedChecks to
  InstallPartitionAllocWithAdvancedChecks.
- Centralized default_dispatch to always be the delegating one when
  USE_PARTITION_ALLOC_AS_MALLOC is true.
- Simplified namespaces and flattened buildflag checks in partition_alloc_support.cc.
- Removes the GN arg from partition_alloc.gni.
- Removes the ENABLE_ALLOCATOR_SHIM_PARTITION_ALLOC_DISPATCH_WITH_ADVANCED_CHECKS_SUPPORT buildflag.
- Simplifies the build configuration in BUILD.gn.
- Replaces buildflag usages with USE_PARTITION_ALLOC_AS_MALLOC.
- Simplifies default dispatch definition in allocator_shim_default_dispatch_to_partition_alloc.cc.

BYPASS_RECITATION_REASON=False positive due to moving code?

Change-Id: I9ffcd70c1d136faead40c458d205261788b8053c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7519087
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Commit-Queue: Mikihito Matsuura <mikt@google.com>
Cr-Commit-Position: refs/heads/main@{#1578524}
NOKEYCHECK=True
GitOrigin-RevId: babe45ae0924ac77cd45098e57f5077ae064501b