[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>

shared/java/javacommon.gradle

@@ -1,6 +1,5 @@
 apply plugin: 'maven-publish'
 apply plugin: 'java-library'
-//apply plugin: 'net.ltgt.errorprone'
 apply plugin: 'jacoco'
 
 def baseArtifactId = project.baseId
@@ -110,9 +109,6 @@ dependencies {
     testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.4.2'
 
     devImplementation sourceSets.main.output
-
-    //errorprone 'com.google.errorprone:error_prone_core:2.3.2-SNAPSHOT'
-    //errorproneJavac 'com.google.errorprone:error_prone_core:2.3.1'
 }
 
 task run(type: JavaExec) {