Wait in loop

Follow-up from commit I1b32d9e069a7b10664065e138c75c6bfe7928f23,
the wait() not in a loop could be errorprone if the call to wait()
unblocks because of a spurious wakeup.

Test: atest CtsNetTestCases:NetworkStatsManagerTest
Bug: 289112440

Change-Id: Ic20ea5a42f3125668f80b13285b43146ae929cbc