Bump version to 2.2.21-beta and update fastlane metadata

fastlane/metadata/android/en-US/changelogs/79.txt

+## Fixed
+* Don't scroll to selected list item if it didn't change. (#989)
+* Revert "Remove unneeded try catch in voice command funtionality (#899)" (#984)
+* Clear webviews before being re-used (#982)

mobile/build.gradle

@@ -31,8 +31,8 @@ android {
         applicationId "org.openhab.habdroid"
         minSdkVersion 14
         targetSdkVersion 27
-        versionCode 78
-        versionName "2.2.20-beta"
+        versionCode 79
+        versionName "2.2.21-beta"
         multiDexEnabled true
         testInstrumentationRunner 'android.support.test.runner.AndroidJUnitRunner'
     }