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/exifinterface/build.gradle b/exifinterface/build.gradle
index 3e034b1..14c5192 100644
--- a/exifinterface/build.gradle
+++ b/exifinterface/build.gradle

@@ -1,4 +1,6 @@
-apply plugin: android.support.SupportAndroidLibraryPlugin
+plugins {
+    id("SupportAndroidLibraryPlugin")
+}
 
 dependencies {
     api project(':support-annotations')
@@ -17,4 +19,5 @@
     publish true
     inceptionYear '2016'
     description 'Android Support ExifInterface'
+    legacySourceLocation true
 }