Make nist-sip compile against public SDK APIs

nist-sip is the only dependency of the ext.jar that
wasn't explicitly building against public SDK
APIs. This change enables us to enforce use of
public SDKs in Android's ext.jar.

Bug: 113148576
Test: make ext
Change-Id: I7d38d35f9069f60af68b64e3bac816476780b39c
1 file changed