Gradle hack to always use same build tools in subprojects

.travis.yml

@@ -11,7 +11,7 @@ before_install:
 
     # Install required Android components.
     #- echo "y" | android update sdk -a --filter build-tools-19.1.0,android-19,platform-tools,extra-android-support,extra-android-m2repository --no-ui --force
-    - ( sleep 5 && while [ 1 ]; do sleep 1; echo y; done ) | android update sdk --no-ui --all --force --filter build-tools-21.1.2,build-tools-21.1.1,build-tools-19.1.0,android-21,android-19,platform-tools,extra-android-support,extra-android-m2repository
+    - ( sleep 5 && while [ 1 ]; do sleep 1; echo y; done ) | android update sdk --no-ui --all --force --filter build-tools-21.1.2,android-21,android-19,platform-tools,extra-android-support,extra-android-m2repository
 install: echo "Installation done"
 script:
     - ./gradlew assemble -S -q

build.gradle

@@ -38,3 +38,34 @@ project(':extern:spongycastle') {
         test.enabled = false
     }
 }
+
+// Copied from https://gitlab.com/fdroid/fdroidclient/blob/master/build.gradle#L144
+subprojects {
+
+    // This is the hacky way which we force the subprojects to use the same build tools:
+    // http://stackoverflow.com/a/21032272
+    afterEvaluate {
+        if ( it.hasProperty( 'android' ) ) {
+            android {
+
+                // The android build task only lets you configure the buildToolsVersion once, so if
+                // we execute the closure below to configure our subprojects, it will fail when it
+                // hits the second subproject. Therefore, we will only do it once, and I guess the
+                // android plugin will re-use the existing value I set.
+                // https://android.googlesource.com/platform/tools/build/+/master/gradle/src/main/groovy/com/android/build/gradle/BaseExtension.groovy
+                try {
+                    buildToolsVersion '21.1.2'
+                    logger.info("Set buildToolsVersion to '21.1.2'")
+                } catch (GradleException e) {
+                    logger.info("Tried to set the buildToolsVersion, however we were not allowed to: $e.message")
+                }
+
+                // don't abort build on lint errors
+                // http://stackoverflow.com/a/25149514
+                configure(android.lintOptions) {
+                    abortOnError false
+                }
+            }
+        }
+    }
+}