Fix race condition in NsdManagerTest

NsdManagerTest relies on the lastMessage variable to assert the
connection of a client to the service. The asserts in test are done on a
different thread, although the variable was not qualified volatile,
which creates the chance to read a stale value. This patch fixes this
race by marking the variable volatile.

Bug: 32561414
Bug: 62044295
Test: NsdManagerTest works
Change-Id: I0bc2cd8059c6ff8b78a2dda0ba8d6bc7e13ef951