Add ON CONFLICT statement to Prompts migration

This avoids crashes when, for some reason, the Prompts table has multiple
entries for the same day. There's no harm dropping duplicate entries as we fetch
prompts and overwrite frequently.

Author: Thomas Horta

fluxc/src/main/java/org/wordpress/android/fluxc/persistence/WPAndroidDatabase.kt

@@ -338,7 +338,9 @@ abstract class WPAndroidDatabase : RoomDatabase() {
                             "respondentsCount, attribution, respondentsAvatars, answeredLink) " +
                             "SELECT id, siteLocalId, text, date, isAnswered, " +
                             "respondentsCount, attribution, respondentsAvatars, " +
-                            "'$tagPrefix' || id FROM BloggingPrompts"
+                            "'$tagPrefix' || id FROM BloggingPrompts " +
+                            "WHERE true " +
+                            "ON CONFLICT DO NOTHING"
                     )
 
                     execSQL("DROP TABLE `BloggingPrompts`")