fastboot: don't use sparse_file_import_auto() in load_buf_fd()

load_buf_fd() attempts to find the size of the file that it is about
to load by first calling sparse_file_import_auto() then using
sparse_file_len() upon success or falling back to the file size on the
filesystem on failure.

This is problematic however as sparse_file_import_auto() creates a
sparse_file out of the normal file, but does not resparse it, so an
assertion fails during the sparse_file_len() call.

This is fixed by using sparse_file_import() instead.  This will fail
in the case that the file is not sparse and the call to
sparse_file_len() will be properly skipped.

Bug: 140538105
Test: flash blueline factory image with assertions enabled in
      libsparse/sparse.cpp

Change-Id: I0283be33563a3301ce5b09bde41105a20f91086c