Let the user choose where to export vcards to 1/3

Instead of always using external storage.

Also we change the suggested export filename
from something like 00001.vcf to just contacts.vcf.
Since we don't know the destination directory
anymore untill after it is selected, we can't check
if there is already an exported vcf file there in.
The Activity handling ACTION_CREATE_DOCUMENT handles
file name collisions itself anyway.

Screenshots https://goto.google.com/b22208705

Bug 22208705

Change-Id: I17bac2f483e200c13adf1429c0eade17e4d2908a