implementing minimumFreeSizeTest function

This function tests whether the requested size for installation is greater
than MinimumFreeSize (250MB) else it will disable the install button and
change its text to 'Not enough space (< 250MB)'. The earlier Toast which does
this work is been removed.

fixes #117 https://github.com/guardianproject/lildebi/issues/117
restore image_path from the Image path EditText box

when we uncheck 'Install on Internal Storage' we have to replace image
path from Image path Edit text box, instead of default value
fix crash on resuming InstallActivity when using internal storage

If we resume InstallActivity keeping the Image size black we get NumberFormatException
Change NativeHelper.limitTo4GB after '4GB Image Limit' Setting changed