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.

Bug: 38499889
Test: adb shell data/local/tmp/VtsHalRadioV1_0TargetTest
Change-Id: I491db829cca607fad9f6a21987de92269d115253
diff --git a/radio/1.0/vts/functional/Android.bp b/radio/1.0/vts/functional/Android.bp
index 7808de1..5403971 100644
--- a/radio/1.0/vts/functional/Android.bp
+++ b/radio/1.0/vts/functional/Android.bp
@@ -27,7 +27,8 @@
            "radio_hidl_hal_test.cpp",
            "radio_hidl_hal_voice.cpp",
            "radio_response.cpp",
-           "VtsHalRadioV1_0TargetTest.cpp"],
+           "VtsHalRadioV1_0TargetTest.cpp",
+           "vts_test_util.cpp"],
     shared_libs: [
         "libbase",
         "liblog",
@@ -51,7 +52,8 @@
     srcs: ["sap_callback.cpp",
            "sap_hidl_hal_api.cpp",
            "sap_hidl_hal_test.cpp",
-           "VtsHalSapV1_0TargetTest.cpp"],
+           "VtsHalSapV1_0TargetTest.cpp",
+           "vts_test_util.cpp"],
     shared_libs: [
         "libbase",
         "liblog",