Add sleep time to start TestActivity

To eliminate flakiness in GameSessionTrampolineActivityTest,
we add sleep time to wait the actvity to be fully launched.

Bug: 245615459
Test: abtd result https://android-build.googleplex.com/builds/abtd/run/L15600000961510101
Change-Id: I33016f439b7d4d48a4a050ecfdff68a8b7e5800e