Protect SparseImage._GetRangeData() with lock

The generator function is not thread safe and is prone to race
conditions. This CL uses a lock to protect this generator and loose the
locks elsewhere, e.g. 'WriteRangeDataToFd()'.

Bug: 71908713
Test: Generate an incremental package several times for angler 4208095 to 4442250.
Change-Id: I9e6f0a182a1ba7904a597f403f2b12fe05016513