Update Android-side Gradle stuff + dependencies + upgrade all Flutter packages to latest

android/build.gradle.kts

@@ -0,0 +1,25 @@
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+        maven {
+            // [required] background_fetch
+            url = uri("${project(":background_fetch").projectDir}/libs")
+        }
+    }
+}
+
+val newBuildDir: Directory = rootProject.layout.buildDirectory.dir("../../build").get()
+rootProject.layout.buildDirectory.value(newBuildDir)
+
+subprojects {
+    val newSubprojectBuildDir: Directory = newBuildDir.dir(project.name)
+    project.layout.buildDirectory.value(newSubprojectBuildDir)
+}
+subprojects {
+    project.evaluationDependsOn(":app")
+}
+
+tasks.register<Delete>("clean") {
+    delete(rootProject.layout.buildDirectory)
+}