chore(mobile): bump to 0.0.51 and Android versionCode 18

0.0.50 is closed on the App Store and shipped as mobile-android-v0.0.50
with versionCode 17, so both values are consumed. Fastlane fails the iOS
release when the resolved version is not higher than the closed train.

mobile/app.json

@@ -2,7 +2,7 @@
   "expo": {
     "name": "Orca",
     "slug": "orca-mobile",
-    "version": "0.0.50",
+    "version": "0.0.51",
     "orientation": "default",
     "icon": "./assets/icon.png",
     "userInterfaceStyle": "automatic",
@@ -75,7 +75,7 @@
       "allowBackup": false,
       "permissions": ["RECORD_AUDIO", "MODIFY_AUDIO_SETTINGS"],
       "package": "com.stably.orca.mobile",
-      "versionCode": 17,
+      "versionCode": 18,
       "googleServicesFile": "./google-services.json"
     },
     "plugins": [