[incfs] Use a more efficient getBlockCount() for incfs v2

v2 IncFS driver gives a very lightweight function to check the
loading progress on a file, use it instead of counting the
filled ranges

+ remove the unused mockable toString(IncFsFileId)

Bug: 183067554
Test: atest IncrementalServiceTest
Change-Id: Icd3bd891d671b27654f4194787a15a00cba1eb80