Merge Flatfoot and Support library plugins

- Moves the Android Library project setup into one place
- Add an option for legacy source paths (support lib projects)
- Enables error-prone for flatfoot projects.
- Removes verbose setup of flatfoot android libraries
- Moves to referencing plugins via new plugins { id("foo") } api.

Test: ./gradlew assemble for both projects
Change-Id: I4dab611740353b0825d892169e9e7a96d900796b
diff --git a/annotations/build.gradle b/annotations/build.gradle
index dd4a5bc..2e8fbab 100644
--- a/annotations/build.gradle
+++ b/annotations/build.gradle
@@ -1,4 +1,6 @@
-apply plugin: android.support.SupportJavaLibraryPlugin
+plugins {
+    id("SupportJavaLibraryPlugin")
+}
 
 jar {
     from sourceSets.main.output