API 29: Fix changed nullability and generics

DropBoxManager.getNextEntry: This wasn't supposed to be @NonNull, null indicates all tags.
Parcel.readSparseArray: Made generic, which should be safe at runtime; could be a mild annoyance
                        at build time, but this seems worth it in the long run.

Bug: 123221743
Test: make api-stubs-docs-check-last-released-api
Change-Id: Icf1527518240bfb4ed26ec29525d24f6521fdc56
1 file changed