Auto merge of #21404 - paulrouget:localaar, r=MortimerGoro

Make it possible to use a custom AAR to build servoapp

Fix https://github.com/servo/servo/issues/21402

To use a local AAR, create `support/android/apk/user.properties`:
```
servoViewLocal=/tmp/servo-latest.aar
```

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support/android/apk/servoapp/build.gradle

@@ -123,5 +123,9 @@ android {
 
 dependencies {
     implementation 'com.android.support.constraint:constraint-layout:1.1.2'
-    implementation project(':servoview')
+    if (findProject(':servoview-local')) {
+      implementation project(':servoview-local')
+    } else {
+      implementation project(':servoview')
+    }
 }

support/android/apk/servoview-local/build.gradle

@@ -0,0 +1,12 @@
+configurations.maybeCreate("default")
+
+if (gradle.hasProperty('servoViewLocal')) {
+    def aar = new File(gradle.servoViewLocal)
+    if (aar.exists()) {
+        artifacts.add('default', aar)
+    } else {
+        throw new GradleException('Failed to find ServoView AAR at: ' + gradle.servoViewLocal)
+    }
+} else {
+    throw new GradleException('Local ServoView AAR path not defined')
+}

support/android/apk/settings.gradle

@@ -1 +1,23 @@
-include ':servoapp', ':servoview'
+include ':servoapp'
+
+def userPropertiesFile = new File('user.properties')
+if (userPropertiesFile.exists()) {
+  println("Loading user.properties")
+    def props = new Properties()
+    userPropertiesFile.withInputStream {
+      props.load(it)
+    }
+  props.each { prop ->
+    println(prop.key + " = " + prop.value)
+      gradle.ext.set(prop.key, prop.value)
+  }
+  if (gradle.hasProperty('servoViewLocal')) {
+    println("Using local build of servoview")
+      include ':servoview-local'
+      project(':servoview-local').projectDir = new File('servoview-local')
+  } else {
+    include ':servoview'
+  }
+} else {
+  include ':servoview'
+}