[build] Enable spotbugs (#3601)

Benign spotbugs warnings were suppressed, and all others were fixed. Bug
descriptions are documented here:
https://spotbugs.readthedocs.io/en/stable/bugDescriptions.html

Co-authored-by: Austin Shalit <austinshalit@gmail.com>

wpilibOldCommands/src/main/java/edu/wpi/first/wpilibj/PIDBase.java

@@ -267,21 +267,18 @@ public class PIDBase implements PIDInterface, PIDOutput, Sendable, AutoCloseable
 
       // Ensures m_enabled check and pidWrite() call occur atomically
       m_pidWriteMutex.lock();
+      m_thisMutex.lock();
       try {
-        m_thisMutex.lock();
-        try {
-          if (m_enabled) {
-            // Don't block other PIDController operations on pidWrite()
-            m_thisMutex.unlock();
+        if (m_enabled) {
+          // Don't block other PIDController operations on pidWrite()
+          m_thisMutex.unlock();
 
-            m_pidOutput.pidWrite(result);
-          }
-        } finally {
-          if (m_thisMutex.isHeldByCurrentThread()) {
-            m_thisMutex.unlock();
-          }
+          m_pidOutput.pidWrite(result);
         }
       } finally {
+        if (!m_enabled) {
+          m_thisMutex.unlock();
+        }
         m_pidWriteMutex.unlock();
       }
 

wpilibOldCommands/src/test/java/edu/wpi/first/wpilibj/command/AbstractCommandTest.java

@@ -18,7 +18,7 @@ public abstract class AbstractCommandTest {
     Scheduler.getInstance().enable();
   }
 
-  public class ASubsystem extends Subsystem {
+  public static class ASubsystem extends Subsystem {
     Command m_command;
 
     @Override

wpilibOldCommands/src/test/java/edu/wpi/first/wpilibj/command/ConditionalCommandTest.java

@@ -17,7 +17,6 @@ class ConditionalCommandTest extends AbstractCommandTest {
   MockCommand m_onTrue;
   MockCommand m_onFalse;
   MockSubsystem m_subsys;
-  Boolean m_condition;
 
   @BeforeEach
   void initCommands() {