Add AKEY_ASSIST mapping in native input framework to allow programmatically inject KEY_ASSISTANT.

Currently, the TV input bridge can only inject Linux KEY_SEARCH through injecting Android AKEYCODE_SEARCH; injecting AKEYCODE_ASSIST gets the following error due to lack of mapping.

```TvRemote-native-uiBridge: Received an unknown keycode of 219.```

Adding the mapping functionality will allow product flexibility in virtual keyboard behaviors.

Test: Programmatically inject KeyEvent.KEYCODE_ASSIST through google3/java/com/google/android/tv/remote/service/RemoteUInputBridge.java, and successfully invoked Katniss when Youtube was playing in foreground.
Bug: 173388770
Change-Id: Ic7c0a447996bb0d33babcba10fd5332f9d1b2738