message: pick location of apksigner from PATH if it's not found in the sdk dir
  Summary:
  I don't have apksigned available under $ANDROID_SDK/build-tools/23.0.1/apksigner
  and as a result `//fbandroid/native/redex:fb-inject-debug` crashes. With this
  change I can run the tool with apksigner on PATH, see the test plan.
diff: @@ -71,6 +71,13 @@ def find_android_build_tools():
  return join(build_tools, version)
  +def find_apksigner():
  + candidate = join(find_android_build_tools(), "apksigner")
  + if os.path.exists(candidate):
  + return candidate
  + return shutil.which("apksigner")
  +
  +
  def remove_signature_files(extracted_apk_dir):
  for f in abs_glob(extracted_apk_dir, "META-INF/*"):
  cert_path = join(extracted_apk_dir, f)
  @@ -81,7 +88,7 @@ def remove_signature_files(extracted_apk_dir):
  def sign_apk(keystore, keypass, keyalias, apk):
  subprocess.check_call(
  [
  - join(find_android_build_tools(), "apksigner"),
  + find_apksigner(),
  "sign",
  "--v1-signing-enabled",
  "--v2-signing-enabled",
  