Restrict PI sender BAL privileges by default.

This is a behavior change for an app that
- sends a PendingIntent created by another app
- targets Android U+
- does not explicitly set a value for `isPendingIntentBackgroundActivityLaunchAllowed`

While before U the system would automatically consider the state of the
sending app to allow the PendingIntent to start an activity from the
background, now the system automatically reduces the privilege to start only
foreground services instead.

Test: atest ActivityStarterTests BackgroundActivityLaunchTest
Bug: 244637991
Change-Id: I4860780f2ad4461ac88bf18f9774343b4ba27255