composer: fix 2.4 vts for multi-display

Fix the way the test handles multi-displays by caching
all the hotplug events received when registering the callback

Test: VTS on Pixel 4
Bug: 174174253
Change-Id: I9a69b18bcef0722e603f2ca58cfbd176ea60f5f2
Merged-In: I9a69b18bcef0722e603f2ca58cfbd176ea60f5f2