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