1. Add string for STATE_CANCELED. Lint doesn't like that the
switch statement on the state IntDef doesn't have a case
for STATE_CANCELED. May as well add one, even if we're not
planning on our demo app showing notifications for this
state.
2. Replace non-human-readable error message with one provided
by ErrorMessageProvider.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=184120892
