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

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

This reverts commit 1dfe7be9.

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>