Avoid random twice to generate an address

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