Add _Nonnull annotations to AImageDecoder

Bug: 160984428
Test: AImageDecoderTest
Test: If6ee6314096bf2ded103f502cac52e56da9be5d8

This provides documentation that parameters (and return values) should
not be null, and allows the compiler to catch some errors. Requires an
update to the CTS tests, which test passing null.

Change-Id: Ife5d519def81b3a8845b43aaaa69d26dc2c0ac20