Do not use rootProject directly in Gradle scripts

Summary:
While testing with the RN Nightly versions, I realized we pushed two changes
recently that are not working fine with the gradle setup of our users.

That's becuase we're referencing the `rootProject` directly.
`rootProject` should never be used directly as it resolves to:
- The root of the git repo of `react-native` when building the RN project (so
  `./ReactCommon` exists there).
- The /android folder of users' project when building an app that uses RN (so
  `./ReactCommon` does not exists there).

Changelog:
[Android] [Fixed] - Do not use `rootProject` directly in Gradle scripts

Reviewed By: sshic

Differential Revision: D35444967

fbshipit-source-id: be0508480a08224302168804b6feb52fd604d8db

Author: cortinico

ReactAndroid/build.gradle

@@ -29,6 +29,7 @@ def AAR_OUTPUT_URL = "file://${projectDir}/../android"
 def customDownloadsDir = System.getenv("REACT_NATIVE_DOWNLOADS_DIR")
 def downloadsDir = customDownloadsDir ? new File(customDownloadsDir) : new File("$buildDir/downloads")
 def thirdPartyNdkDir = new File("$buildDir/third-party-ndk")
+def reactNativeRootDir = projectDir.parent
 
 // You need to have following folders in this directory:
 //   - boost_1_63_0
@@ -277,7 +278,7 @@ android {
 
         externalNativeBuild {
             cmake {
-                arguments "-DREACT_COMMON_DIR=${rootProject.projectDir}/ReactCommon",
+                arguments "-DREACT_COMMON_DIR=${reactNativeRootDir}/ReactCommon",
                         "-DREACT_ANDROID_DIR=$projectDir",
                         "-DANDROID_STL=c++_shared",
                         "-DANDROID_TOOLCHAIN=clang",

ReactAndroid/hermes-engine/build.gradle

@@ -13,16 +13,17 @@ plugins {
     id("maven-publish")
 }
 
+def reactNativeRootDir = project(':ReactAndroid').projectDir.parent;
 def customDownloadDir = System.getenv("REACT_NATIVE_DOWNLOADS_DIR")
-def downloadsDir = customDownloadDir ? new File(customDownloadDir) : rootProject.file("sdks/download")
+def downloadsDir = customDownloadDir ? new File(customDownloadDir) : new File(reactNativeRootDir, "sdks/download")
 
 // By default we are going to download and unzip hermes inside the /sdks/hermes folder
 // but you can provide an override for where the hermes source code is located.
 def overrideHermesDir = System.getenv("REACT_NATIVE_OVERRIDE_HERMES_DIR") != null
-def hermesDir = System.getenv("REACT_NATIVE_OVERRIDE_HERMES_DIR") ?: rootProject.file("sdks/hermes")
+def hermesDir = System.getenv("REACT_NATIVE_OVERRIDE_HERMES_DIR") ?: new File(reactNativeRootDir, "sdks/hermes")
 
 def hermesVersion = "main"
-def hermesVersionFile = rootProject.file("sdks/.hermesversion")
+def hermesVersionFile = new File(reactNativeRootDir, "sdks/.hermesversion")
 if (hermesVersionFile.exists()) {
     hermesVersion = hermesVersionFile.text
 }
@@ -34,7 +35,7 @@ def prefabHeadersDir = new File("$buildDir/prefab-headers")
 def skipPrefabPublishing = System.getenv("REACT_NATIVE_HERMES_SKIP_PREFAB") != null
 
 // We inject the JSI directory used inside the Hermes build with the -DJSI_DIR config.
-def jsiDir = rootProject.file("ReactCommon/jsi")
+def jsiDir = new File(reactNativeRootDir, "ReactCommon/jsi")
 
 // The .aar is placed inside the ./android folder at the top level.
 // There it will be placed alongside the React Android .aar