TestKeyStore: add the ability to set certificate serial numbers

Change URLConnectionTest#testHttpsWithCustomTrustManager
to check for specifically set serial numbers.

As of 5a85130cc3ee3df65c3b263773e3649277b37317, the serial numbers
were being generated randomly, and the test was looking for the
value "1", so the test was broken.

(cherry picked from commit a23e1a7d40a0a85f8eed3a94d6b5714f98180a4e)

Bug: 27987415
Change-Id: I91d2726f362a6aab75edbb6b0d133c00ab774e73
2 files changed