I'll answer your question in reverse
Which form is best for android users, a dialog message, a toast message or something else?
To answer this you will need to understand what a toast message is used for and when do you use a dialog message.
- A toast message is used to give a notification about some event or activity which is not critical and needs user interaction. To quote the android developers site
A toast provides simple feedback about an operation in a small popup.
It only fills the amount of space required for the message and the
current activity remains visible and interactive. For example,
navigating away from an email before you send it triggers a "Draft
saved" toast to let you know that you can continue editing later.
Toasts automatically disappear after a timeout.
- Dialog box : To quote the android developers site again :
A dialog is a small window that prompts the user to make a decision or
enter additional information. A dialog does not fill the screen and is
normally used for modal events that require users to take an action
before they can proceed.
Now which one should you use ?
This depends on your app and the need your app has for internet connectivity. If your app is like a ebook reader where you just need internet connectivity to check for updates, then a toast notification might suffice if its not critical to the functioning of the app and the app can just work fine without internet.
However if its an app which needs real time data from the internet (e.g. A news reading app which will not show any data without internet access) then a dialog box might work out better as you need to get your users to acknowledge an action is needed.
Which is the best way to display the internet connectivity message, with or without 'Retry' button? Is not the 'retry'button more intuitive?
I would recommend going for the retry option as the retry button allows them to do another call to see if internet access is available and doesn't require them to close the popup notification and try to refresh the app by any other means to get connectivity back. However do ensure you have a suitable error message which informs them that network access is not available so that users know what they are retrying for.
With regards to your comment, Youtube and Play store do have an alternate method of refresh i.e. pull to refresh and hence they can get away with a error message and expect the user to use the pull to refresh.Google drive can show a toast notification as it has still some locally referenced content and can work without internet for a while.