ANDROID: Set internal vars for 6.13.

The 6.13 merge introduces a few changes that:

- Causes KBUILD_MIXED_TREE related patches to conflict. It turns out
that the best fix is to just copy artifacts to $OUT_DIR directly. Hence,
we need a flag so Kleaf can continue working on 6.12.

- MO is now supported to build external modules in a separate build
directory. However, this causes the VPATH trick to stop working and
requires MO to be set. So we need a flag so that Kleaf can continue to
work on 6.12.

- Employ a hack that temporarily works around the issue in Kbuild where
the pattern matching for the COPY rule doesn't work yet.

Bug: 402231876
Bug: 402888498
Change-Id: I789be4d4c3ab849d7b07337e742217af100ceba0
Signed-off-by: HONG Yifan <elsk@google.com>

build.config.constants

@@ -4,3 +4,6 @@ RUSTC_VERSION=1.82.0
 AARCH64_NDK_TRIPLE=aarch64-linux-android31
 X86_64_NDK_TRIPLE=x86_64-linux-android31
 ARM_NDK_TRIPLE=arm-linux-androideabi31
+KLEAF_INTERNAL_KBUILD_MIXED_TREE_IS_OBJTREE=1
+KLEAF_INTERNAL_EXT_MODULE_SEPARATE_BUILD_DIR=1
+KLEAF_INTERNAL_COPY_RULE_HACK=1