Update AGP to 7.1.2.

AGP 7.1.0 adds maven-publish APIs for making publishing javadocs and sources for Android libraries very easy. We can use that to resolve #32.

Changelog: https://developer.android.com/studio/releases/gradle-plugin#versioning-update.

build.gradle.kts

@@ -4,7 +4,7 @@ buildscript {
         mavenCentral()
     }
     dependencies {
-        classpath("com.android.tools.build:gradle:7.0.4")
+        classpath("com.android.tools.build:gradle:7.1.2")
         classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.6.10")
     }
 }