ANDROID: Move BRANCH back to build.config.common.

Revert "ANDROID: Move BRANCH from build.config.common to .constants."

This reverts commit 1dfe7be90d.

Reason for revert: BRANCH is no longer used anywhere in .bzl / BUILD
files. For dist_dir, we now use out/<target_name>/dist instead of
out/<branch>/dist.

This is one less item in build.config.constants that requires
kernel_toolchain_ext in bzlmod.

Bug: 276493276
Change-Id: I2efb9144b760c58eeec2eca9f11548943a057324
Signed-off-by: Yifan Hong <elsk@google.com>

build.config.common

@@ -1,5 +1,6 @@
 . ${ROOT_DIR}/${KERNEL_DIR}/build.config.constants
 
+BRANCH=android-mainline
 LLVM=1
 DEPMOD=depmod
 CLANG_PREBUILT_BIN=prebuilts/clang/host/linux-x86/clang-${CLANG_VERSION}/bin

build.config.constants

@@ -1,4 +1,3 @@
-BRANCH=android-mainline
 CLANG_VERSION=r510928
 AARCH64_NDK_TRIPLE=aarch64-linux-android31
 X86_64_NDK_TRIPLE=x86_64-linux-android31