Remove TODO(b/347672184) and explain why @Nullable is not used

After a conversation with the API council it was decided that the
benefits of marking the `findViewById(int)` methods `@Nullable` is
greatly outweighed by the cost.

Bug: 347672184
Test: N/A
Change-Id: I79b299e3988f1ed3bf25e2528d8450291f738a1e
5 files changed