FgsBgStartTest: Wait for the startForeground from test app ProcState of the test app could be changed to FGS before calling startForeground(), while if subsequently it's told to stop, the call to startForeground could crash the test app. Bug: 162450085 Test: atest CtsAppTestCases Change-Id: I76e2115292258014dd0e9775b559696795599b37