Fix kotlin nullable errors in HealthFitness

Fix kotlin nullable errors that were exposed by setting the retention
of android.annotation.NonNull and android.annotation.Nullable to
class retention.

Bug: 294110802
Test: builds
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1c0fc0e888d6b4934dbe47bdb0e7dea8433ade62)
Merged-In: I31abf75fac33af973a6cdbe519a64acc8296c1e8
Change-Id: I31abf75fac33af973a6cdbe519a64acc8296c1e8
1 file changed