Add Platform adapter around unbundled conscrypt

This wraps the conscrypt OpenSSLSocketImpl with an adapter that is a
subclass of the platform's OpenSSLSocketImpl in order to support old
code that does casts to the platform OpenSSLSocketImpl in order to set
things like SNI.

Until KK the platform OpenSSLSocketImpl was
org.apache.harmony.xnet.provider.jsse.OpenSSLSocketImpl, in KK it became
com.android.org.conscrypt.OpenSSLSocketImpl. As of L MR1 the platform
HTTP stack no longer casts to the platform OpenSSLSocketImpl and this
work around is not needed on those devices.

Change-Id: I196ad957eabfc70246d9c01aa12855a8eab036f0
diff --git a/build.gradle b/build.gradle
index 12d7b99..a2db9fa 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1,11 +1,19 @@
 apply plugin: 'java'
 
+sourceSets {
+    stub {
+        java.srcDirs = [
+            'src/stub/java'
+        ]
+    }
+}
 // this is the "Unbundled Conscrypt jar"
 sourceSets.main {
     java.srcDirs = [
         'src/main/java',
         'src/compat/java',
     ]
+    compileClasspath += sourceSets.stub.output
 }
 
 compileJava.options.encoding = 'UTF-8'