Upgrade kotlinx-coroutines deps to 1.4.2.

This seems changes Proguard junk to get rid of some warnings
while building:

  R8: Missing class: java.lang.instrument.ClassFileTransformer
  R8: Missing class: sun.misc.SignalHandler

Kotlin/kotlinx.coroutines#2266

(It may only fix ClassFileTransformer; not sure. I don't
seem to see these every time.)

Author: derat

build.gradle

@@ -4,7 +4,7 @@
 buildscript {
   // https://developer.android.com/kotlin/add-kotlin
   ext.kotlin_version = '1.4.30'
-  ext.kotlin_coroutines_version = '1.3.9'
+  ext.kotlin_coroutines_version = '1.4.2'
 
   dependencies {
     // 4.1.0+ requires Gradle 6.5:
@@ -29,9 +29,4 @@ allprojects {
     jcenter()
     mavenCentral()
   }
-
-  // Uncomment to display warnings about unchecked exceptions and deprecated APIs.
-  //tasks.withType(JavaCompile) {
-  //  options.compilerArgs << "-Xlint:unchecked" << "-Xlint:deprecation"
-  //}
 }