Android: Add hooks for NDK to replace CMake Platform/Android* modules

The Android NDK comes with a toolchain file that intends to handle all
Android-related settings and avoid using CMake's own Android support.
However, it includes many settings in the toolchain file itself that are
supposed to belong in `Platform/Android*.cmake` modules.  Normally
third-party platforms can be configured using a toolchain file that sets
`CMAKE_MODULE_PATH` to a directory with `Platform/<os>*.cmake` modules,
but that does not work when CMake comes with builtin modules for that
platform due to policy CMP0017.

Add an undocumented `CMAKE_ANDROID_NDK_HOOK_DIR` variable for use by the
Android NDK's toolchain file to specify a directory with modules that
hook into CMake's modules.  Teach each `Platform/Android*.cmake` module
to include a like-named module from the specified hook directory and
then skip the rest of its logic.  Also offer a per-module option to not
skip the rest of its logic, just in case.

This hook is needed for any `Platform/Android*.cmake` module that is
included by other CMake builtin modules since CMP0017 prevents
`CMAKE_MODULE_PATH` from overriding such includes.

Issue: #18787
NDK-Issue: 463
Status Job ID Name Coverage
  External
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