[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
diff --git a/services/incremental/IncrementalService.cpp b/services/incremental/IncrementalService.cpp
index ebfcc32..34253f9 100644
--- a/services/incremental/IncrementalService.cpp
+++ b/services/incremental/IncrementalService.cpp
@@ -2860,7 +2860,7 @@
     dprintf(fd, "        lastPendingReads: \n");
     const auto control = mService.mIncFs->openMount(mHealthPath);
     for (auto&& pendingRead : mLastPendingReads) {
-        dprintf(fd, "          fileId: %s\n", mService.mIncFs->toString(pendingRead.id).c_str());
+        dprintf(fd, "          fileId: %s\n", IncFsWrapper::toString(pendingRead.id).c_str());
         const auto metadata = mService.mIncFs->getMetadata(control, pendingRead.id);
         dprintf(fd, "          metadataHex: %s\n", toHexString(metadata).c_str());
         dprintf(fd, "          blockIndex: %d\n", pendingRead.block);