Add an API to get recent IMS disconnect reasons

ImsManager will store the last 16 disconnect reasons when
onImsDisconnect callback is invoked. This information can be
accessed by the clients of ImsManager using the new
getRecentImsDisconnectReasons API

Bug: 34694758
Test: Ensure that the new API returns IMS disconnect reasons
in chronological order after running a few scenarios where IMS
gets connected and disconnected.

Change-Id: Iff5edeb9bba8898349ffce64f5975f5b1717cf76