Contacts: Fix security exception when export to .vcf file

When export contacts to external .vcf file, it will display
"contacts keeps stopping". Temporary read and write permissions
must be revoked when activity destroy. At this scenario, it
still access this external .vcf file after ExportVcardActivity
destroy, then cause security exception.

Obtain external .vcf file name before activity destroy and
issue resolved.

Test: built the Contacts app installed and verified that it doesn't
crash when exporting.
Bug: 154946972

Change-Id: I855bde6d1147db7fc40a54be43e30a37da8e545b