Elaborate on the reason of FGS background start.

shouldAllowFgsFeatureLocked used to return boolean, now return an
integer that specify the reason of denial/allowance.

If FGS startForeground() is allowed in bg proc state because the list of
exemptions, still generate the Slog.wtf() log messages so we have a
complete list of bg started FGS.

Bug: 157473819
Test: adb logcat | grep "Background started FGS"
atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java

Change-Id: I21cbaa1d6c44e7c252b8cc91fe258065d0b2ccaf