sync: include TARGET_RELEASE when constructing smart sync target.

When using the smart sync option, we try to construct the target that
was "lunched" from the TARGET_PRODUCT and TARGET_BUILD_VARIANT envvars.

However, an android target is now made of three parts,
{TARGET_PRODUCT}-{TARGET_RELEASE}-{TARGET_BUILD_VARIANT}.

I am leaving the option of creating a target if a TARGET_RELEASE is not
specified in case there are other consumers who depend on that option.

BUG=b:358101714
TEST=./run_tests
TEST=smart sync on android repo and manually inspecting
smart_sync_override.xml

Change-Id: I556137e33558783a86a0631f29756910b4a93d92
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/436977
Tested-by: Yiwei Zhang <yiwzhang@google.com>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>