don't run clang tidy on external unless requested

Bug: 268668248
Test: compare number of tidy actions before and after change:
  WITH_TIDY=1 bb aquery --config=android 'mnemonic(ClangTidy, deps(//packages/modules/NeuralNetworks/apex:com.android.neuralnetworks))' --output=summary
Test: verify that Bazel tidy actions don't have a large diff from Soong tidy actions:
  (add -v to Ninja invocation in ui/build/ninja.go [and disable fifo file])
  WITH_TIDY=1 DISABLE_ARTIFACT_PATH_REQUIREMENTS=true m com.android.adbd 2>&1 | tee soong_build-tidy.adbd &&
  WITH_TIDY=1 DISABLE_ARTIFACT_PATH_REQUIREMENTS=true m com.android.adbd --bazel-mode-staging 2>&1 | tee mixed_build-tidy.adbd &&
  grep -Po "prebuilts/clang/host/linux-x86/clang-r475365b/bin/clang-tidy.sh \K\S+" soong_build-tidy.adbd | sort -u > soong-clang-tidy.adbd &&
  grep -Po "prebuilts/clang/host/linux-x86/clang-r475365b/bin/clang-tidy.sh \K\S+" mixed_build-tidy.adbd | sort -u > mixed-clang-tidy.adbd &&
  vimdiff soong_build-tidy.adbd mixed_build-tidy.adbd
Change-Id: I5193cd58b1b40bcdf9b955e45d20eb722d8292a8