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/fragment/build.gradle b/fragment/build.gradle
index 3a86f38..0bfe003 100644
--- a/fragment/build.gradle
+++ b/fragment/build.gradle
@@ -1,4 +1,6 @@
-apply plugin: android.support.SupportAndroidLibraryPlugin
+plugins {
+    id("SupportAndroidLibraryPlugin")
+}
 
 dependencies {
     api project(':support-compat')
@@ -23,5 +25,6 @@
     publish true
     inceptionYear '2011'
     description "The Support Library is a static library that you can add to your Android application in order to use APIs that are either not available for older platform versions or utility APIs that aren\'t a part of the framework APIs. Compatible on devices running API 14 or later."
+    legacySourceLocation true
 }