Add CTS test case for the FGS start grace period after being in BG.

If a visibile activity switches to background, within a certain
amount of time, the app would still be eligible to start FGS from BG.

Bug: 182796375
Test: atest CtsAppTestCases:ActivityManagerFgsBgStartTest
Change-Id: I783d9497b1afaa0492d483134ef84346477561f2