Delete blobs if there are no active leases on them.

+ Fix a bug in how sessions belonging to uninstalled pkgs
  are removed.

Bug: 148637068
Test: atest services/tests/servicestests/src/com/android/server/blob/BlobStoreManagerServiceTest.java
Change-Id: I98cd9e974fb1a46bb5868a644b1a0d98ff61eb57