Update BlobStoreMS to augment storage stats with blobs data.

- Any pending sessions data is attributed to the apps
  which contributed them.
- Any commited blobs data is attributed to the app which
  has a lease on it. If multiple apps have lease on a blob, don't
  attribute the blob to those apps for now.
- Remove StorageStatsAugmenter.augmentStatsForUser as it
  is not used for anything currently.
- Fix an issue in how we override existing committers and leasees.

Bug: 148694869
Test: atest cts/tests/BlobStore/src/com/android/cts/blob/BlobStoreManagerTest.java
Test: atest tests/tests/os/src/android/os/storage/cts/StorageStatsManagerTest.java
Test: atest hostsidetests/appsecurity/src/android/appsecurity/cts/StorageHostTest.java
Test: manual
Change-Id: Ia4af0a2549c75db66741f2d1979de95d2d150bc8