Separate annotation processors and include in java as provided

Summary:
With the current setup, the final aar contains `com.facebook.proguard.annotation.DoNotStrip`.  This is not needed (it is only used for proguard), and thus should not get published.

We move proguard annotations into its own project for this, and place it into `java/proguard-annotations`. Here we adopt the gradle convention of `src/main` to make for a nice, clean (one line!) gradle script.  As a different subproject, we can include this to `:yoga` as a `provided` dependency now, which doesn't include it in the output artifact.

Reviewed By: emilsjolander

Differential Revision: D4666572

fbshipit-source-id: a0cb26cb6c264065a0bd355b7d72ba02e3759560

java/build.gradle

@@ -44,6 +44,7 @@ preBuild.dependsOn buckBuildAndCopy
 dependencies {
     compile(name: 'jsr305')
     compile(name: 'soloader-0.1.0', ext: 'aar')
+    provided(project(':yoga:proguard-annotations'))
 }
 
 task sourcesJar(type: Jar) {

java/proguard-annotations/build.gradle

@@ -0,0 +1 @@
+apply plugin: 'java'