build: Allow building release artifacts in-tree

This change avoids all need for changing any file under
VCS to insert signing keys and configs for release builds.

Example contents of keystore.properties

```
// Location of keystore, relative to module build.gradle,
// in this case, of the app module
storeFile=../wireguard.jks
storePassword=b3ty0uc4nth4xxth1s
keyAlias=wireguard
keyPassword=4ndr01dsux
```

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

.gitignore

@@ -13,3 +13,5 @@ build/
 *.class
 *.dex
 *.iml
+*.jks
+keystore.properties

app/build.gradle

@@ -1,5 +1,9 @@
 apply plugin: 'com.android.application'
 
+// Create a variable called keystorePropertiesFile, and initialize it to your
+// keystore.properties file, in the rootProject folder.
+final def keystorePropertiesFile = rootProject.file("keystore.properties")
+
 android {
     buildToolsVersion '27.0.3'
     compileOptions {
@@ -17,6 +21,28 @@ android {
         versionCode 422
         versionName '0.0.20180605'
     }
+    // If the keystore file exists
+    if (keystorePropertiesFile.exists()) {
+        // Initialize a new Properties() object called keystoreProperties.
+        final def keystoreProperties = new Properties()
+
+        // Load your keystore.properties file into the keystoreProperties object.
+        keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
+
+        signingConfigs {
+            release {
+                keyAlias keystoreProperties['keyAlias']
+                keyPassword keystoreProperties['keyPassword']
+                storeFile file(keystoreProperties['storeFile'])
+                storePassword keystoreProperties['storePassword']
+            }
+        }
+    }
+    buildTypes {
+        release {
+            if (keystorePropertiesFile.exists()) signingConfig signingConfigs.release
+        }
+    }
     externalNativeBuild {
         cmake {
             path 'tools/CMakeLists.txt'