tunnel: switch publishing to MavenCentral

Since MavenCentral requires GPG signed artifacts, we'll need to configure
some things on the machine running the deployment. Specifically, these
three Gradle properties need to be set (preferably in ~/.gradle/gradle.properties)

```
signing.keyId= // Duh
signing.password= // I have absolutely no idea how this will work with HSMs
signing.secretKeyRingFile= // $HOME/.gnupg/... you know the deal
```

The BINTRAY_USER and BINTRAY_KEY variables are replaced by SONATYPE_USER
and SONATYPE_PASSWORD to better reflect their new contents.

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

tunnel/publish.gradle

@@ -1,4 +1,5 @@
 apply plugin: 'maven-publish'
+apply plugin: 'signing'
 
 afterEvaluate {
     publishing {
@@ -41,11 +42,11 @@ afterEvaluate {
         }
         repositories {
             maven {
-                name = "bintray"
-                url = uri("https://api.bintray.com/maven/wireguard/wireguard-android/wireguard-android/;publish=1;override=0")
+                name = "sonatype"
+                url = uri("https://oss.sonatype.org/service/local/staging/deploy/maven2/")
                 credentials {
-                    username = hasProperty('BINTRAY_USER') ? getProperty('BINTRAY_USER') : System.getenv('BINTRAY_USER')
-                    password = hasProperty('BINTRAY_KEY') ? getProperty('BINTRAY_KEY') : System.getenv('BINTRAY_KEY')
+                    username = hasProperty('SONATYPE_USER') ? getProperty('SONATYPE_USER') : System.getenv('SONATYPE_USER')
+                    password = hasProperty('SONATYPE_PASSWORD') ? getProperty('SONATYPE_PASSWORD') : System.getenv('SONATYPE_PASSWORD')
                 }
             }
         }
@@ -70,3 +71,7 @@ android.libraryVariants.all { variant ->
         }
     }
 }
+
+signing {
+    sign publishing.publications
+}