Add paging-rxjava3 artifact

This adds an rxjava3 extensions artifact for paging v3. It's ported from the rx2 artifact, but with all the deprecated APIs from that artifact removed.

I had to update the coroutines version to 1.3.7 (technically 1.3.6 was when it was introduced, but with a serious issue affecting android lint that is fixed in 1.3.7). I don't know how to get those prebuilts updates included in my CL though, not sure if it's necessary. Let me know.

RelNote: "Add RxJava3 compatibility wrappers for PagingSource and
RemoteMediator"
Fixes: 161480176
Test: ./gradlew paging:paging-rxjava3:test

Change-Id: I49ef38fde9b84f92dd272ed0b1cd1719fbbf1761
diff --git a/paging/rxjava3/build.gradle b/paging/rxjava3/build.gradle
new file mode 100644
index 0000000..d7895df
--- /dev/null
+++ b/paging/rxjava3/build.gradle
@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+
+import androidx.build.AndroidXExtension
+import androidx.build.LibraryGroups
+import androidx.build.Publish
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
+import static androidx.build.dependencies.DependenciesKt.*
+
+plugins {
+    id("AndroidXPlugin")
+    id("com.android.library")
+    id("kotlin-android")
+}
+
+dependencies {
+    api(project(":paging:paging-common"))
+    api("androidx.arch.core:core-runtime:2.1.0")
+    api(RX_JAVA3)
+    implementation(KOTLIN_STDLIB)
+    implementation(KOTLIN_COROUTINES_RX3)
+
+    testImplementation(project(":internal-testutils-common"))
+    testImplementation(project(":internal-testutils-paging"))
+    testImplementation(JUNIT)
+    testImplementation(KOTLIN_TEST)
+    testImplementation(KOTLIN_COROUTINES_TEST)
+}
+
+androidx {
+    name = "Android Paging-RxJava3"
+    publish = Publish.SNAPSHOT_AND_RELEASE
+    mavenGroup = LibraryGroups.PAGING
+    inceptionYear = "2020"
+    description = "Android Paging-RxJava3"
+    url = AndroidXExtension.ARCHITECTURE_URL
+}
+
+// Allow usage of Kotlin's @OptIn.
+tasks.withType(KotlinCompile).configureEach {
+    kotlinOptions {
+        freeCompilerArgs += ["-Xopt-in=kotlin.RequiresOptIn"]
+    }
+}