Fix FLAG_EXEMPT_FROM_APP_STANDBY check

It was checking with the time constraint, but the sync manager
uses FLAG_EXEMPT_FROM_APP_STANDBY for back-off syncs too, so
it was actually valid.

Instead let's just make sure periodic syncs don't have it.

Bug: 73119752
Test: atest CtsJobSchedulerTestCases
Change-Id: Ib87680fcef0707440a186848db076c4524a632af