Fixed BackgroundInstallControlServiceTest flaky case.

The test case relied on consecutive calls of System.currentTimeMillis()
and SystemClock.uptimeMillis() with the difference be at most 1msec.
Which is not a practical time window for consistency. Adding a large
buffer instead.

Test: atest FrameworksServicesTests:com.android.server.pm.BackgroundInstallControlServiceTest#testHandleUsageEvent_fallsBackToAppInfoTimeWhenHistoricalSessionsNotFound --iterations 10
Bug: 326107161
Change-Id: I4d46f9500ffa553d83a324ebac11b4d259ee852f