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

This allows Bazel to load the value of $BRANCH in order
to determine the value of --dist_dir of copy_to_dist_dir
statically.

Test: TH
Bug: 229268271

Change-Id: Iff759b8188360ea1b2bc204d29750eece9095582
Signed-off-by: Yifan Hong <elsk@google.com>
(cherry picked from commit 75020bfb)