Revert part of previous change.

allowBgActivityStartsOnServiceStart() should be called after it is
certain that the service start is allowed.

Bug: 157473819
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Change-Id: Ifcde7f18ea07e9cece84134a48848f84aa6db4ca