Question: I know this is not a technical related question.But i do not have any idea.I am going to develop a project which should be supported in both phone and tablet.Is it possible having one project working for both tablet and phone ?

Answer: 
Please, use MySQL.

There is almost no excuse to use Access backend. 

Like you said, Access has very poor connection abilities (every time the database is open, a connection is created as long as the program is still opened). Also, it has poor backup options (do you really want to make copies of the file every day?).

Also, Access has a "repair and compress database" option (for MDB, not sure about the newer format). It has a poor record of data consistency from what I experience with MDBs daily.

MySQL gives you a reliable data store, connections can be pooled properly, it has transaction support, and you can also run proper backups on it.

You can have a MySQL backend, and use Access forms as a frontend. I expect this should be a good enough compromise, and lets you scale it easily.
I have seen Access database have real poor security and which is terribly slow. If at all you intend to open your access database after a few days of running in production, you will see the how slow it can be. 

Though it looks like you are new to MySQL, you will not need much of a learning curve to work with it. MySQL have lot of users that you will get a good support for you MySQL related questions also. And the best of all you will be able to learn MySQL, which i am sure you will use later for other applications you create.