Fix GenerateRandom() for tests

Currently we reset the seed every time GenerateRandom is called. In test
this might be in the same instant, which result in same random number
being generated twice. Make sure we initialize seed just once.

Also, add test for ECDH key computation that uses the GenerateRandom

Bonus: fixing GenerateRandom() revealed an error in
pairing_handler_le_unittest, since generated numbers are not the same.

Test: added
Bug: 142341141
Change-Id: Ib8e498ceea32445eec0afa73b69b67ee2e748838