Bump checkstyle to 6.12.1

Makes the existing checks a lot better (we were on 5.x) and allows us to
add checks that require newer versions.

F-Droid/build.gradle

@@ -182,6 +182,10 @@ android {
     }
 }
 
+checkstyle {
+    toolVersion = '6.12.1'
+}
+
 task checkstyle(type: Checkstyle) {
     configFile file("${project.rootDir}/config/checkstyle/checkstyle.xml")
     source 'src', 'test/src'

Privileged-Extension/build.gradle

@@ -35,6 +35,10 @@ android {
 
 }
 
+checkstyle {
+    toolVersion = '6.12.1'
+}
+
 task checkstyle(type: Checkstyle) {
     configFile file("${project.rootDir}/config/checkstyle/checkstyle.xml")
     source 'src'