Add perf test to measure duration of blob store digest computation.

+ Move BlobStoreTestUtils from cts/ to frameworks/base/tests/.

Bug: 148898557
Test: atest ./apct-tests/perftests/blobstore/src/com/android/perftests/blob/BlobStorePerfTests.java
Change-Id: I2de155d0c0c1fb602c57353ba4819bdc9cda8c0a