Updates in pattern generation

[lee30sonia]

Some minor updates in simulation pattern generation:

• Avoid using 0 as the default random seed as it creates the same patterns for the first two PIs.
• Randomize don't-care bits in bitpacked_simulator to maximize the randomness of the patterns (values generated by the SAT solver may not be really random even if the randomize option is turned on for the solver).

[hriener]

Thank you!