build: migrate from cordova to capacitor

Closes #85

android/variables.gradle

@@ -0,0 +1,14 @@
+ext {
+    minSdkVersion = 21
+    compileSdkVersion = 30
+    targetSdkVersion = 30
+    androidxActivityVersion = '1.2.0'
+    androidxAppCompatVersion = '1.2.0'
+    androidxCoordinatorLayoutVersion = '1.1.0'
+    androidxCoreVersion = '1.3.2'
+    androidxFragmentVersion = '1.3.0'
+    junitVersion = '4.13.1'
+    androidxJunitVersion = '1.1.2'
+    androidxEspressoCoreVersion = '3.3.0'
+    cordovaAndroidVersion = '7.0.0'
+}