Update BlobStoreManager javadoc.

- Indicate that blob could be deleted even if the app
  acquires a lease.
- Indicate the meaning of passing 0 as blob expiry time.

Bug: 143559646
Test: atest cts/tests/BlobStore/src/com/android/cts/blob/BlobStoreManagerTest.java
Change-Id: I4deb72cb5b5500d42825781e329e9b2ec9839c7b