impl(common): improve InvocationIdGenerator seed (#13711)

Use `MakeDefaultPRNG()` to initialize the PRNG (pseudo-random number generator)
in `InvocationIdGenerator`.  This function consumes the same amount of
entropy from the OS as the previous call, but uses `std::seed_seq` to
fill the initial state of the PRNG. AFAIK, `std::seed_seq` produces
better distributed seeds from the same entropy.

Author: coryan

google/cloud/internal/invocation_id_generator.cc

@@ -24,7 +24,7 @@ namespace internal {
 
 // Initialize the random bit source with some minimal entropy.
 InvocationIdGenerator::InvocationIdGenerator()
-    : generator_(std::random_device{}()) {}
+    : generator_(MakeDefaultPRNG()) {}
 
 std::string InvocationIdGenerator::MakeInvocationId() {
   // A retry id is supposed to be a UUID V4 string. We assume you have read the