Update checkstyle to 10.0 and fix various related issues

- Put checkstyle files into checkstyle/ subfolder so that the gradle task does not implicitly depend on the whole project, fixing many warnings during build and possibly increasing build performance.
- Remove unused SuppressionXpathFilter from config file.
- Remove outdated suppressions from suppressions file.

app/build.gradle

@@ -98,7 +98,7 @@ android {
 }
 
 ext {
-    checkstyleVersion = '9.3'
+    checkstyleVersion = '10.0'
 
     androidxLifecycleVersion = '2.3.1'
     androidxRoomVersion = '2.4.2'
@@ -121,7 +121,7 @@ configurations {
 }
 
 checkstyle {
-    getConfigDirectory().set(rootProject.file("."))
+    getConfigDirectory().set(rootProject.file("checkstyle"))
     ignoreFailures false
     showViolations true
     toolVersion = checkstyleVersion