Pass START_ACTIVITIES_FROM_BACKGROUND permission to instrumentation

test.

So the instrumentation can have the same permssion as app and it can
skip the FGS background start while-in-use restriction.

Also reuse the ProcessRecord.mAllowBackgroundActivityStartsTokens flag.

Bug: 136219221
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Change-Id: I7d8708945ffe79b83825f8edb688078706c46770