Allow certain apps to access blobs across users.

BlobStoreManagerService is updated to store only
one copy of any blob on device and to allow apps
with new ACCESS_BLOBS_ACROSS_USERS priv permission
to access blobs commited by their instances on other
users.

Bug: 175844032
Test: atest --test-mapping apex/blobstore
Change-Id: If6d10d90b17bde33baf9e83eeaae75d09d0b50ae