Update Java linters and fix new PMD errors (#4157)

PMD requires that variables only initialized in the constructor be
final. The compiler errors if those final variables aren't guaranteed to
be initialized, so extra else branches were added to ensure that.

PMD also requires that classes with only private constructors be final.
The equivalent C++ classes were finalized as well, except for
TimeInterpolatableBuffer because it doesn't expose factory functions.

shared/java/javastyle.gradle

@@ -2,7 +2,7 @@ if (!project.hasProperty('skipJavaFormat')) {
     apply plugin: 'checkstyle'
 
     checkstyle {
-        toolVersion = "9.2"
+        toolVersion = "10.1"
         configDirectory = file("${project.rootDir}/styleguide")
         config = resources.text.fromFile(new File(configDirectory.get().getAsFile(), "checkstyle.xml"))
     }
@@ -10,7 +10,7 @@ if (!project.hasProperty('skipJavaFormat')) {
     apply plugin: 'pmd'
 
     pmd {
-        toolVersion = '6.41.0'
+        toolVersion = '6.44.0'
         consoleOutput = true
         reportsDir = file("$project.buildDir/reports/pmd")
         ruleSetFiles = files(new File(rootDir, "styleguide/pmd-ruleset.xml"))