Move checkstyle config to buildscripts/

Checkstyle configuration is just noise in the root directory. Neither
users nor developers need to look at it often.

android-interop-testing/build.gradle

@@ -24,7 +24,7 @@ subprojects {
     apply plugin: "checkstyle"
 
     checkstyle {
-        configFile = file("$rootDir/../checkstyle.xml")
+        configFile = file("$rootDir/../buildscripts/checkstyle.xml")
         toolVersion = "6.17"
         ignoreFailures = false
         if (rootProject.hasProperty("checkstyle.ignoreFailures")) {

build.gradle

@@ -239,7 +239,7 @@ subprojects {
     }
 
     checkstyle {
-        configFile = file("$rootDir/checkstyle.xml")
+        configFile = file("$rootDir/buildscripts/checkstyle.xml")
         toolVersion = "6.17"
         ignoreFailures = false
         if (rootProject.hasProperty("checkstyle.ignoreFailures")) {

buildscripts/checkstyle.xml

@@ -21,7 +21,7 @@
     <property name="severity" value="error"/>
 
     <module name="Header">
-      <property name="headerFile" value="${rootDir}/checkstyle.license"/>
+      <property name="headerFile" value="${rootDir}/buildscripts/checkstyle.license"/>
       <property name="ignoreLines" value="2"/>
       <property name="fileExtensions" value="java"/>
     </module>