* C#: Fixed possible conflicts between usings and user-supplied namespace
C#: Added the global qualifier to using directives to prevent possible conflicts with the user-supplied namespace. Also prevents unintentional type hiding. Resolves issue #4242.
* Updated C# generated code files
* Fix flatc generating incorrect java/c# code for namespace_test2.fbs. In code for TableInC, method referToA2() returned type SecondTableInA instead of NamespaceA.SecondTableInA.
* Updated generated code.
* Fixed indendation.
Similar to what protobufs does with its `Message` interface, introduce here such interface and create a generic `GetRootAs` method to deserialize a flatbuffer.
Also fixed most codegenerators using the wrong namespace when multiple
namespace were used in a file, with some files not being generated.
Change-Id: Ib42969221239d7244e431cbd667ef69200fc415f
Tested: on Linux.
Bug: 29338474