API 29: Fix changed nullability

DropBoxManager.getNextEntry: This wasn't supposed to be @NonNull, null indicates all tags.

Bug: 123221743
Test: make api-stubs-docs-check-last-released-api
Change-Id: Icf1527518240bfb4ed26ec29525d24f6521fdc56
Merged-In: Icf1527518240bfb4ed26ec29525d24f6521fdc56
(cherry picked from commit 41a6d51c017c4b3c33fdc7f87b6a9216f3a824a9)
1 file changed