Change getHost annotation to match setHost. (#938)

* Change getHost annotation to match setHost.

Author: rlazo

firebase-firestore/src/main/java/com/google/firebase/firestore/FirebaseFirestoreSettings.java

@@ -146,7 +146,7 @@ public Builder setCacheSizeBytes(long value) {
     }
 
     /** @return the host of the Cloud Firestore backend. */
-    @Nullable
+    @NonNull
     public String getHost() {
       return host;
     }
@@ -156,9 +156,7 @@ public boolean isSslEnabled() {
       return sslEnabled;
     }
 
-    /**
-     * @return boolean indicating whether local persistent storage is enabled or not.
-     */
+    /** @return boolean indicating whether local persistent storage is enabled or not. */
     public boolean isPersistenceEnabled() {
       return persistenceEnabled;
     }