libavb_user: convert avb_ops_user.c to .cpp

fs_mgr_get_entry_for_mount_point() is switching to a C++ only API (using
std::string). Do a minimum conversion in libavb_user to adopt that
change.

Bug: 67024518
Test: avbctl get-verification
Test: avbctl disable-verification
Test: avbctl enable-verification
Test: All unit tests pass

Change-Id: I755d86d517409ba00020708784854416b3c7b2bd