Require new fields to have usable defaults.

If an old server sends a parcelable, new fields must be able to
represent the empty or null type so that transactions from old
interfaces aren't automatically rejected.

Bug: 146215188
Test: local: `m $(allmod | grep -P ndk_platform$)`
Test: TH does checkbuild
Test: aidl_unittests

Change-Id: I55e8c5bc324aa4bbb62a035b3e7ff6db566227f4
2 files changed