[CI] Add gradle remote build cache (bitfireAT/davx5#752)

* [CI] Add gradle remote build cache

* Update workflow

* Don't cache local build cache; pre-populate configuration cache

* Allow configuration caching of tasks

* Free some disk space before running instrumented tests; cache whole .android (not only .android/avd)

* Allow branches to update configuration cache

* Use dry run to pre-populate configuration cache

* Test runs: don't cache

* Fix remote build cache configuration for non-CI builds

* Add comment

settings.gradle.kts

@@ -19,4 +19,18 @@ dependencyResolutionManagement {
     }
 }
 
+// use remote build cache, if configured
+if (System.getenv("GRADLE_BUILDCACHE_URL") != null) {
+    buildCache {
+        remote<HttpBuildCache> {
+            url = uri(System.getenv("GRADLE_BUILDCACHE_URL"))
+            credentials {
+                username = System.getenv("GRADLE_BUILDCACHE_USERNAME")
+                password = System.getenv("GRADLE_BUILDCACHE_PASSWORD")
+            }
+            isPush = true        // read/write
+        }
+    }
+}
+
 include(":app")