Support methods taking compile-time constant fields in Access Paths.

[lazaroclapp]

This builds upon PR #285 by adding support to code like:

if (x.get(ZERO) != null && x.get(ZERO).foo() != null) {
   return x.get(ZERO).foo().bar();
}

Where ZERO is a compile-time evaluated constant. That is,
a static final variable or field of primitive or string type.

[lazaroclapp]

Trivia: As might be inferred from the branch name, this is some intermediate work I ran into while trying to support treating io.grpc.Metadata as if it were a Map (even though it doesn't implement the map interface, it has a similar containsKey(...) and get(...) logic). This patch is not needed for that change, but it seems useful enough in general.

[coveralls]

Pull Request Test Coverage Report for Build #580

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 89.186%

---

Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/uber/nullaway/dataflow/AccessPath.java	10	94.65%

Totals
Change from base Build #579:	0.02%
Covered Lines:	3505
Relevant Lines:	3930

---

💛 - Coveralls

[msridhar]

LGTM!