Promote getLeasedBlobs() to be a public API.

This would allow apps to know which blobs they currently
hold a lease on and release any if they are running out
of lease quota.

Bug: 152219149
Test: atest --test-mapping apex/blobstore
Change-Id: I0141cfcd7cdbb83dbc75be11463ab90ee3c3f7cc
3 files changed