nested types: GenerateHeaderIncludes()

Visitor-based function gathers all include files for a header with
nested types.

However GenerateHeader() doesn't support nested types yet. Upcoming
changes will make it to generate class decl for each nested type
recursively.

Bug: 182508839
Test: aidl_unittests
Test: tests/golden_test.sh check
Change-Id: I2bb6b160a6a99fef7576c4c22f10e6015f3dc5c0
6 files changed