Add persistence for user history dictionary

User history dictionary was not flushing to disk on close, added that logic so (unless there's a crash) the user history should be persisted.

app/src/main/java/org/dslul/openboard/inputmethod/latin/DictionaryFacilitatorImpl.java

@@ -225,6 +225,7 @@ public class DictionaryFacilitatorImpl implements DictionaryFacilitator {
 
     @Override
     public void onFinishInput(Context context) {
+        flushDictionaries();
     }
 
     @Override
@@ -459,6 +460,18 @@ public class DictionaryFacilitatorImpl implements DictionaryFacilitator {
         }
     }
 
+    public void flushDictionaries() {
+        final DictionaryGroup dictionaryGroupToClose;
+        synchronized (mLock) {
+            dictionaryGroupToClose = mDictionaryGroup;
+            mDictionaryGroup = new DictionaryGroup();
+        }
+        for (final String dictType : ALL_DICTIONARY_TYPES) {
+            ExpandableBinaryDictionary dict = dictionaryGroupToClose.getSubDict(dictType);
+            if (dict != null) dict.asyncFlushBinaryDictionary();
+        }
+    }
+
     @UsedForTesting
     public ExpandableBinaryDictionary getSubDictForTesting(final String dictName) {
         return mDictionaryGroup.getSubDict(dictName);