Parse AIDL string constants

Parse declarations of string constants as part of an interface:

interface IFoo {
  const String MY_CONSTANT = "value";
}

These constants are restricted to be printable ASCII characters < 0x80.

A follow on CL will introduce the actual code generation.
While here, fix a bug where an empty interface definition generates
an empty Call enum in C++.

Bug: 28233277
Change-Id: I9a46962d601ad39d4024ad783ca3dbc8e60a15c7
Test: Introduced new end to end tests.
10 files changed