Access process fields only when valid

In some cases we can tear down ServiceRecord bookkeeping before its
hosting ProcessRecord has been established.  Make sure that we don't
attempt to reference ProcessRecord information unless we've already
established that it's available.  In this case, the service can't be in
the FGS state until the app's code has begun running and has called
startForeground(), so checking `isForeground` provides the guarantee.

Fixes: 197070543
Test: atest CtsAppTestCases:android.app.cts.ServiceTest
Change-Id: I429c31c7ea87625b380a1ed70b6a1d9afa45bdb3