Add nullability annotations to public methods of ConnManager.

This is mainly for Kotlin compatibility.
https://android-developers.googleblog.com/2018/08/android-pie-sdk-is-now-more-kotlin.html

Bug: 122437895
Test: no logic changes
Change-Id: Ic1b9a774aee0099397b1ccfd958ff3b61d69a7fb
1 file changed