API revisions in SmsManager and TelephonyManager

- Make SmsManager#sendMultipartTextMessage public (the one that was
previously @SystemApi)
- Split TelephonyManager#getDefaultRespondViaMessageApplication into a
pure getter and a getter/updater

Bug: 147299011
Test: atest TelephonyManagerTest

Change-Id: I44160920a66b7ebd18eca3ad4773b5a144738b39