ANDROID: Add optional __get xattr method paired to __vfs_getxattr

Add an optional __get xattr method that would be called, if set, only
in __vfs_getxattr instead of the regular get xattr method.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 133515582
Bug: 136124883
Bug: 129319403
Change-Id: If9f6cf3e0d964e77af769244bb1e8f6aee4f4445