Enable publishing sources and javadocs for bdk-android.

This resolves bitcoindevkit#32.
Using new API from AGP 7.1.0 to generate and publish Android sources and javadocs.

https://developer.android.com/studio/releases/gradle-plugin#publish-javadoc-jar
https://developer.android.com/studio/releases/gradle-plugin#publish-sources-jar

android/build.gradle.kts

@@ -23,6 +23,13 @@ android {
             proguardFiles(file("proguard-android-optimize.txt"), file("proguard-rules.pro"))
         }
     }
+
+    publishing {
+        singleVariant("release") {
+            withSourcesJar()
+            withJavadocJar()
+        }
+    }
 }
 
 dependencies {