Random select started prefix range

Currently, PrivateAddressCoordinator always selects the /24 prefix
from the 192.168.x.x/16 range first. If the upstream is also a /24
prefix in the 192.168.x.x/16 range, there's a 1/256 chance of a
conflict (192.168.0.x/24 ~ 192.168.255.x/24). Since 192.168.x.x/16
is a popular range for home and small business networks, we should
randomize the starting prefix range to reduce the chance of a
conflict. The /8, /12, and /16 range selection rates are 0.39%,
5.86%, and 93.7%, respectively (see the inline comments for how
the rates are calculated).

The chance of selecting a /24 prefix is:
- 10.0.0.0/8: 1/(256*256) * 93.7% = 0.001429%
- 172.16.0.0/12: 1/(16*256) * 5.86% = 0.00143%
- 192.168.0.0/16: 1/256 * 0.39% = 0.001523%

Note: This change is currently turned off by the
tether_force_random_prefix_base_selection flag. We'll run experiments
to make sure tethering stays reliable with this change enabled.

Bug: 312647670
Test: atest TetheringTests
Change-Id: Iea03fdcf0fccad95410e79ae87fcb046d75da457