QueueTest: Only set promise value when the key is removed

Setting the promise before removing the value was making some tests
flaky because they assumed that the value was removed when the promise
returned which happened most of the time until it didn't

Bug: 264500085
Test: bluetooth_test_gd_unit64 --gtest_filter=QueueTest.* --gtest_repeat=-1
Change-Id: I951e21b78d971966a40d3c53aa1c26caa532f0df