Fix flaky UdpEncapsulationSocket test

This commit reduces the flakiness of the
testOpenAndCloseUdpEncapsulationSocket by retrying up to three times.
Unfortunately, testing port-selected socket creation is racy against
other applications. This helps to handle the same race condition as done
in IpSecService#bindToRandomPort

Bug: 128024100
Test: 200x runs of testOpenAndCloseUdpEncapsulationSocket
Change-Id: I7e036ce821019dbac6c50899bd0894e89d2fe82a
Merged-In: Idf040a67e53d9b9ec6e6c647ce24f8ada501d355
Merged-In: Iad9aea4b42cd8b31a5a2659bb9cb54dd1c64e8b7
(cherry picked from commit 614ab3dd)