Cache Android builds on GitHub Actions (#1922)

Summary:
Builds up a file with checksums of all `build.gradle` files. Deliberately excludes the `gradle.properties` which changes all the time. The cache shouldn't require nuking except in some rare circumstances.

Pull Request resolved: https://github.com/facebook/flipper/pull/1922

Test Plan: From 41 minutes down to 17: https://github.com/passy/flipper-1/actions/runs/561501374

Reviewed By: mweststrate

Differential Revision: D26427242

Pulled By: passy

fbshipit-source-id: ff1d7370d477c9d7b57c62082ec985108a5ba698

scripts/checksum-android.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+RESULT_FILE=$1
+
+if [ -f "$RESULT_FILE" ]; then
+  rm "$RESULT_FILE"
+fi
+touch "$RESULT_FILE"
+
+checksum_file() {
+  openssl sha256 "$1" | awk '{print $2}'
+}
+
+FILES=()
+while read -r -d ''; do
+  FILES+=("$REPLY")
+done < <(find . -type f \( -name "build.gradle*" -o -name "gradle-wrapper.properties" \) -print0)
+
+# Loop through files and append checksum to result file
+for FILE in "${FILES[@]}"; do
+  checksum_file "$FILE" >> "$RESULT_FILE"
+done
+
+sort "$RESULT_FILE" -o "$RESULT_FILE"