Fix ReactAndroid build break. (facebook#22377)

Summary:
During C++ build, we need the libjsc.so.
  But arm64-v8a and x86_64 libjsc.so are in different path and this error raised:

    Android NDK: ERROR:/home/circleci/react-native/ReactAndroid/build/third-party-ndk/jsc/Android.mk:jsc: LOCAL_SRC_FILES points to a missing file
    /opt/ndk/android-ndk-r17c/build/core/prebuilt-library.mk:45: *** Android NDK: Aborting    .  Stop.
    Android NDK: Check that /home/circleci/react-native/ReactAndroid/build/third-party-ndk/jsc/jni/arm64-v8a/libjsc.so exists  or that its path is correct

  The commit moves prebuilt libjsc.so into
  ReactAndroid/src/main/jni/third-party/jsc/jni and let ndkbuild script find the prebuilt libjsc.so.
  For AAR packaging, modify the jniLibs so that gradle android library plugin could find the prebuilt libjsc.so as well.
Pull Request resolved: facebook#22377

Differential Revision: D13166556

Pulled By: hramos

fbshipit-source-id: 61daaede7defbc66491a3e2f20058e7d248ba13e

ReactAndroid/build.gradle

@@ -162,7 +162,7 @@ task prepareJSC(dependsOn: dependenciesPath ? [] : [downloadJSCHeaders]) << {
     copy {
         from zipTree(configurations.compile.fileCollection { dep -> dep.name == 'android-jsc' }.singleFile)
         from dependenciesPath ? "$dependenciesPath/jsc-headers" : {downloadJSCHeaders.dest}
-        from 'src/main/jni/third-party/jsc/Android.mk'
+        from 'src/main/jni/third-party/jsc'
         include 'jni/**/*.so', '*.h', 'Android.mk'
         filesMatching('*.h', { fname -> fname.path = "JavaScriptCore/${fname.path}"})
         into "$thirdPartyNdkDir/jsc";
@@ -289,7 +289,7 @@ android {
 
     sourceSets.main {
         jni.srcDirs = []
-        jniLibs.srcDir "$buildDir/react-ndk/exported"
+        jniLibs.srcDirs = ["$buildDir/react-ndk/exported", 'src/main/jni/third-party/jsc/jni']
         res.srcDirs = ['src/main/res/devsupport', 'src/main/res/shell', 'src/main/res/views/modal', 'src/main/res/views/uimanager']
         java {
             srcDirs = ['src/main/java', 'src/main/libraries/soloader/java', 'src/main/jni/first-party/fb/jni/java']