[minfs] Make fsck unfriend minfs class.

In order to make the code more encapsulated , this change removes the "friend"
relation between fsck and minfs class as well as inode-manager class. We use the interfaces
added as part of inspector changes and instead of fsck directly accessing the private members,
we use these Interfaces instead.

Testing: No behavior change. Merely converted private members to be accessed via public interfaces instead of directly via friend relation.
Change-Id: I8fae7acf6d59b47ad2c3376a91d602a127e34de7