The code currently calls RandomInt twice to generate an address.
For example, if you want to generate the address 192.168.A.B,
RandomInt will be called twice to generate the values for A and B.
To save resources, we can avoid calling RandomInt twice by reusing
the same random value with different masks for A and B.
Before:
  -  RandomInt() & 0xff00 -> A
  -  RandomInt() & 0x00ff -> B
After:
  -  RandomInt() & 0xffff -> A.B

Bug: 312647670
Test: atest TetheringTests
Change-Id: Iee49af3d82781309f78fe66ab07937b8137fa259