ThreadPoolAvailableThreads fix.

ThreadPoolAvailableThreads would fail occasionally due to slow threads.
Sometimes a thread might not arrive on time by the time of asserting.
Increased the wait time to 500ms to allow for threads to all arrive and
prevent a flake.

Bug: 270513342
Test: atest BinderLibTest#ThreadPoolAvailableThreads --iterations 1000
Change-Id: I5924a939304802ce618f52c993cb995c735ba21a