Enhance build process (#994)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/994

After building yoga aar, we found several issues:
1. More dynamic so files. This is bad as lower-end Android devices cannot load that many sos.
2. Size increase.
3. (Minor) The libs are stored in asset folder rather than "libs/".

We apply the following optimizations:
1. Remove dependency on memalign16 (this is brought in by a pure header dependency jni-hack);
2. Enable native relinker to remove unused symbols in the so files.
3. Link yogacore statically to reduce size churn.

Reviewed By: SidharthGuglani

Differential Revision: D20808623

fbshipit-source-id: 6c6bbd4f71b6bf6ad272ec05dd56696ddb14a8e0

android/BUCK

@@ -7,6 +7,7 @@ load("//tools/build_defs/oss:yoga_defs.bzl", "ANDROID_JAVA_TARGET", "ANDROID_RES
 
 yoga_android_aar(
     name = "android",
+    enable_relinker = True,
     manifest_skeleton = "src/main/AndroidManifest.xml",
     visibility = [
         "PUBLIC",