Upgrade Kotlin (to 1.3), Coroutines, Gradle and Android gradle plugin.

Kotlin:                1.2.71 -> 1.3.50
Coroutines:            0.30.2 -> 1.3.1
Gradle:                4.6    -> 5.4.1
Android gradle plugin: 3.2.1  -> 3.5.0

This brings us down to *one* experimental coroutine API, and we've
opted in to using it in just *one* place.

(The fact that the API to opt-in to using an experimental API in a
specific place is *also* experimental surely will not come back to
bite us later.)

app/build.gradle

@@ -228,14 +228,12 @@ dependencies {
 
     implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 
-    final coroutines_version = '0.30.2'
-    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$coroutines_version"
-    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core-common:$coroutines_version" // TODO: hack!
+    final coroutines_version = '1.3.1'
     implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:$coroutines_version"
 }
 
 buildscript {
-    ext.kotlin_version = '1.2.71'
+    ext.kotlin_version = '1.3.50'
     repositories {
         mavenCentral()
     }
@@ -248,10 +246,9 @@ repositories {
     mavenCentral()
 }
 
-kotlin {
-    experimental {
-        coroutines 'enable'
-    }
+// See https://kotlinlang.org/docs/reference/experimental.html#experimental-status-of-experimental-api-markers
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.AbstractKotlinCompile).all {
+    kotlinOptions.freeCompilerArgs += ["-Xuse-experimental=kotlin.Experimental"]
 }
 
 androidExtensions {