Addresses further review comments from ag/8000041

Including:
 - An API to opt out of Int/StringDefs generation on per-field basis
 - A way to customize Builder
 - Non-optional fields are passed in Builder constructor
 - Various adjustments to SampleDataclass examples, as requested

Test: . $ANDROID_BUILD_TOP/frameworks/base/tests/Codegen/runTest.sh
Change-Id: I32d2eec52f05d505ff07779d923e4793d3036579