Properly decode a uid from the globalObjId in invites

* Meeting invitations in EAS include a globalObjId.  It turns out
  that this id is EITHER the actual uid (if Exchange created it)
  or a wrapper for the actual uid (if some other client created it)
* To find out which case we're dealing with, we have to look at
  the base64 decoded string for the magic "vCal-Uid" substring
* If it's there, we pull the real uid out of the decoded string
* Otherwise, we build a hex strong from the decoded bytes
* Write unit test for this process

Bug: 2598201
Change-Id: I1cc40af6d1e45be44c19465eb8a4c31851ec8157
3 files changed