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- Remove outdated suppressions from suppressions file.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

checkstyle/checkstyle.xml

@@ -22,7 +22,7 @@
 
     <!-- https://checkstyle.org/config_filters.html#SuppressionFilter -->
     <module name="SuppressionFilter">
-        <property name="file" value="${config_loc}/checkstyle-suppressions.xml" />
+        <property name="file" value="${config_loc}/suppressions.xml" />
         <property name="optional" value="true"/>
     </module>
 
@@ -180,12 +180,5 @@
         <module name="UpperEll"/>
 
         <module name="SuppressWarningsHolder" />
-
-        <!-- https://checkstyle.org/config_filters.html#SuppressionXpathFilter -->
-        <module name="SuppressionXpathFilter">
-            <property name="file" value="${org.checkstyle.sun.suppressionxpathfilter.config}"
-                      default="checkstyle-xpath-suppressions.xml" />
-            <property name="optional" value="true"/>
-        </module>
     </module>
 </module>

checkstyle/suppressions.xml

@@ -7,18 +7,10 @@
     files="LocalItemListAdapter.java"
     lines="232,304"/>
 
-  <suppress checks="FinalParameters"
-    files="InfoListAdapter.java"
-    lines="253,325"/>
-
   <suppress checks="EmptyBlock"
     files="ContentSettingsFragment.java"
     lines="227,245"/>
 
-  <suppress checks="LineLength"
-    files="WebMWriter.java"
-    lines="156,158"/>
-
   <suppress checks="FileLength"
     files="Player.java"/>