Generate serial number randomly in VTS unit test

Any fixed integer can be returned by vendor's APIs response.
Ideally it should be a random value for testing a vendor library.

Applying this to all the unit tests serial number.

Cherry-pick from:
  https://googleplex-android-review.git.corp.google.com/#/c/2330821/

Bug: 38499889
Test: adb shell data/local/tmp/VtsHalRadioV1_0TargetTest
Change-Id: I491db829cca607fad9f6a21987de92269d115253
Merged-In: I491db829cca607fad9f6a21987de92269d115253
(cherry picked from commit 0e8a3be4)