Stabilize hci_layer_test

Currently this test fails around 1 every 10 runs, with null pointer
exception.

Bug: 177907117
Tag: #feature
Test: atest bluetooth_test_gd --gtest_filter=HciTest.receiveMultiple* --gtest_repeat=100
Change-Id: I58de30ae493dc1a2764e36a4451906218632fd47