Currently, OpenAddedFile in smbprovider_test.cc uses SmbProvider (variable smbprovider_) to open a file in a given path. We want to refactor this function to use FakeSambaInterface (variable fake_samba_) instead so we can get stop creating an unnecessary protobuf and also remove the mount_id parameter in the function.

Things that will be changing:
1. Use FakeSambaInterface#OpenFile instead of SmbProvider#OpenFile.
2. Remove mount_id parameter from OpenAddedFile and other calling methods.

Thing to note:
OpenAddedFile currently takes in a |path| variable, which is a relative path. With this change, it will need to take in a full path since FakeSambaInterface expects full path to the entries. There are helper methods such as GetDefaultFullPath() that converts a relative path and prepends the default mount root to the beginning of the path

Test with:
cros_workon_make --board=${BOARD} smbprovider --test