Save contacts to local file and share

We extend ExportVCardActivity in order to reuse VCarcService
to write contacts data to vcf file in background. This change
gets rid of TransactionTooLargeException and provides better
user experience, since we write data in background.

The current UX design is: user will receive Toast message
once export file is ready, and then user needs to tap the
notification to share contacts.

Bug 22083005

Change-Id: I7d7142f3037b1a0647d185d477365df8f2994271
(cherry picked from commit e6c7494bbafe1bef1187245510b1ec0beba6ce10)