Enforce @NonNull package name for onMediaKeyEventSessionChanged

Previously, listeners could receive unexpected null package names.

Test: atest MediaSessionManagerTest
Bug: 274082657
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f8fd9975fba6ca0299b7b31f69422c90412790dd)
Merged-In: I0fdada5ed6d27a02ee0c1ab58423238c7651b5de
Change-Id: I0fdada5ed6d27a02ee0c1ab58423238c7651b5de