[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/ServiceWrappers.h b/services/incremental/ServiceWrappers.h
index 2f3eb24..a787db5 100644
--- a/services/incremental/ServiceWrappers.h
+++ b/services/incremental/ServiceWrappers.h
@@ -84,6 +84,8 @@
             void(std::string_view root, std::string_view backingDir,
                  std::span<std::pair<std::string_view, std::string_view>> binds)>;
 
+    static std::string toString(FileId fileId);
+
     virtual ~IncFsWrapper() = default;
     virtual Features features() const = 0;
     virtual void listExistingMounts(const ExistingMountCallback& cb) const = 0;
@@ -99,7 +101,6 @@
     virtual incfs::RawMetadata getMetadata(const Control& control, FileId fileid) const = 0;
     virtual incfs::RawMetadata getMetadata(const Control& control, std::string_view path) const = 0;
     virtual FileId getFileId(const Control& control, std::string_view path) const = 0;
-    virtual std::string toString(FileId fileId) const = 0;
     virtual std::pair<IncFsBlockIndex, IncFsBlockIndex> countFilledBlocks(
             const Control& control, std::string_view path) const = 0;
     virtual incfs::LoadingState isFileFullyLoaded(const Control& control,