Fix jsc-android not found if building from source (#24547)

Summary:
If an app [builds from RN source](https://facebook.github.io/react-native/docs/building-from-source), there was an error for jsc-android not found.
It is a side effect of my previous [JSC as node dependency change](8e37585)
For building from RN source case, the jsc-android is located at `/path/to/app/node_modules/jsc-android`.
Original gradle task will try to find it at `/path/to/app/node_modules/react-native/ReactAndroid/../node_modules/jsc-android`, as ReactAndroid project path was being override inside node_modules.
The change fixes the building from source case.

N/A
This change does not need to publish into changelog, as it is a master branch building fix.
Pull Request resolved: #24547

Differential Revision: D15044703

Pulled By: cpojer

fbshipit-source-id: a7d824b1a14064d46c4a2ec9ea28255179174c83

ReactAndroid/build.gradle

@@ -136,11 +136,16 @@ task prepareGlog(dependsOn: dependenciesPath ? [] : [downloadGlog], type: Copy)
 // Create Android.mk library module based on jsc from npm
 task prepareJSC {
     doLast {
-        def jscPackageRoot = fileTree("$projectDir/../node_modules/jsc-android/dist")
-        def jscAAR = jscPackageRoot.matching({ it.include "**/android-jsc/**/*.aar" }).singleFile
+        def jscPackageRoot = file("$projectDir/../node_modules/jsc-android/dist")
+        if (!jscPackageRoot.exists()) {
+            // For an app to build from RN source, the jsc-android is located at /path/to/app/node_modules
+            // and $projectDir may located at /path/to/app/node_modules/react-native/ReactAndroid
+            jscPackageRoot = file("$projectDir/../../jsc-android/dist")
+        }
+        def jscAAR = fileTree(jscPackageRoot).matching({ it.include "**/android-jsc/**/*.aar" }).singleFile
         def soFiles = zipTree(jscAAR).matching({ it.include "**/*.so" })
 
-        def headerFiles = jscPackageRoot.matching({ it.include "**/include/*.h" })
+        def headerFiles = fileTree(jscPackageRoot).matching({ it.include "**/include/*.h" })
 
         copy {
             from(soFiles)