Elaborate on ProcessRecord's BG-FGS-launch ability.

ProcessRecord.mAllowStartFgs used to be a boolean type, now make it
a "@ActiveServices.FgsFeatureRetCode int" type so it can carry the
reason code that BG-FGS-launch is enabled.

Bug: 171305836
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Change-Id: I124ed9c9de564f5b797f8cb3461878046ca3d5fa