Use generic type for JsonParser

The android emulator team uses a newer libchrome library. The json read
method now returns an Optional type, vs. a unique_ptr.

We now let the compiler infer the type and rely on the operators to do
the right thing.

Change-Id: I0a668115659d6508344190783b7dc55d84374f15
Test: System compiles.
Bug: 182487647