Enhance error data structure.

[nimakarimipour]

This PR is build upon #103.

In NullAway, if the error is resolvable and involved a pseudo-assignment of a @Nullable expression into a @NonNull target, the location of that target is serialized. This information is enough to generate a fix resolving the reported error. Currently we only store the location of the @Nonnull element in Error data structure. This PR updates Error data structure to store the actual fix which can resolve the error.

Changes in this PR

• Update Error data structure to store the resolving fixes.
• Creates fixes according to the information serialized for the reporting error.

[nimakarimipour]

@msridhar @lazaroclapp Thank you for the review, landing it now.