Return randomly generated session ids from createSession().

Bug: 149324538
Test: atest cts/tests/BlobStore/src/com/android/cts/blob/BlobStoreManagerTest.java
Test: atest ./services/tests/mockingservicestests/src/com/android/server/blob/BlobStoreManagerServiceTest.java
Change-Id: I889c0cc0ebe8baef65f741a2f15c1e3201aec7d2
2 files changed