Fix two issues with jBitmap.LoadFromFile method

[Codereamp]

This patch tries to solve two issues with the jBitmap.LoadFromFile method.

• Without it, it's impossible to correctly load a file when it's in some of the subfolders of the known folder. For example, if I would like to load /storage/emulated/0/myrootfolder/mysubfolder/myfile.jpg, the current (incorrect) implementation will try to load /storage/emulated/0/myfile.jpg since the ExtractFileName would ignore the subfolder part. In case of this example, the final value for FImageName will be myrootfolder/mysubfolder/myfile.jpg. Probably this is not technically "image name", but it's correct to always restore the file location from the field data.
• In case fullFileName is not recognized, the current version (incorrect) will force leading slash. So if I pass /someaccessiblefolder/somefile.jpg, the current implementation won't find a known directory in this path and will pass //someaccessiblefolder/somefile.jpg to jni. So, this commit addresses also this issue.

The change with underscore symbol for FWidth, FHeight (visible in diff) is because while testing I dealt with an older implementation of jBitmap when the fields were called differently.

[jmpessoa]

Thank you!

[Codereamp]

@jmpessoa: Thank you for your work )