Relax the check waiting for the storage clear signal.

There is some data left over after the storage is cleared.
Allow the test to proceed even if this is the case as small
delta is already accounted for in the test verifications.

Bug: 380350458
Test: atest tests/BlobStore/src/com/android/cts/blob/BlobStoreManagerTest.java
Flag: TEST_ONLY
Change-Id: I210941d49ad39cd7265cc5b1a0616476b99b60fc