Remove explicit initialization of default values

Default values are false for boolean, 0 for int and char, and null
for object references.

Explicitly initializing a variable to the default value causes it
to be initialized twice.

Enable the Checkstyle warning, and fix occurences of it.

Change-Id: I1971e438fc506c18458b1402fcb01e43fa61ff8c

tools/checkstyle.xml

@@ -95,6 +95,7 @@ edited to remove noisy warnings.
     </module>
     <module name="RedundantImport"/>
     <module name="RedundantModifier"/>
+    <module name="ExplicitInitialization"/>
   </module>
   <module name="FileTabCharacter">
     <property name="severity" value="ignore"/>