vts: Upsize 8bit uniform_int_distribution

uniform_int_distribution<a> for types sizeof(a) < 2 are
not valid by the C++ library standard. Newer versions of LLVM
(particularly spurred on by ChromeOS toolchain changes)
require at least std::uniform_int_distribution<uint16_t>.

This is a required change for rolling LLVM to r458507.
This is necessary, but may not be sufficient to resolve
the issue.

Bug: 231351802
Test: mma
Change-Id: I04c3cc91507f3467c432b9a25effdac3f5fb56f3