Java: fix unsigned default value code generation and add 'L' suffix for long default value (#4051)

* Java: emit "signed" equivalent of unsigned default value in generated code and add "L" suffix to long default value.

* Updated generated code

* Only convert ulong to "signed" equivalent. ubyte and ushort don't need specific handling as "user facing" type is int. uint need 'L' suffix as "user facing" type is long.

* Added missing cast to primitive type of default value which is in "user facing" type in builder.add<type>() calls.

* Do not cast default value to actual type in C#.