Wait for the profile to not be empty in BootImageProfileTest

Keep trying to force profile saves until it's not empty. This handles
the case when the system server isn't started enough to save a profile
yet. This can be affected by boot time fluctuations if we don't wait.

Test: atest BootImageProfileTest
Bug: 139883463

Change-Id: I98b359412f919bd64c2cd2a5b7d6659d1cef113d