Revert "Revert "Add automatic default value for char-type field"" am: 6466daabe5 am: af03551d36

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/tools/aidl/+/16379730

Change-Id: Ifd72972f7917bfa1d71bc747019ffcbe386b2fdb