Fix Launcher3 gradle issue with src_plugins.

src_plugins was not being pulled in as a source preventing building
any launcher variant APK from the IDE.  This fixes that.

Test: Build APK for all variants w/ other fixes included
Change-Id: Ifc23d15492de0ac9dad37fda621958eb64046b0e
diff --git a/build.gradle b/build.gradle
index 738ce33..c1f373e 100644
--- a/build.gradle
+++ b/build.gradle
@@ -70,7 +70,7 @@
     sourceSets {
         main {
             res.srcDirs = ['res']
-            java.srcDirs = ['src']
+            java.srcDirs = ['src', 'src_plugins']
             manifest.srcFile 'AndroidManifest-common.xml'
             proto {
                 srcDir 'protos/'