Migrate from AndroidManifest to build.gradle for namespace

AGP has deprecated use of packageName in AndroidManifest and
instead wants everyone to use `android { namespace = "foo" }`
in build.gradle files.

Test: ./gradlew tasks -> still passes
Change-Id: I4fa03cacc7214011f3797c36dcb76c2984c7bf52
diff --git a/paging/paging-rxjava2/build.gradle b/paging/paging-rxjava2/build.gradle
index 771eee2..64d838a 100644
--- a/paging/paging-rxjava2/build.gradle
+++ b/paging/paging-rxjava2/build.gradle
@@ -55,6 +55,10 @@
     legacyDisableKotlinStrictApiMode = true
 }
 
+android {
+    namespace "androidx.paging.rxjava2"
+}
+
 // Allow usage of Kotlin's @OptIn.
 tasks.withType(KotlinCompile).configureEach {
     kotlinOptions {