Use real caller UID to determine if BAL privileges may be granted

Instead of the calling UID (the process that wrapped the broadcast in a `PendingIntent`) use the real calling UID (the process that actually sent the `PendingIntent`) to determine if granting BAL (background activity launch) privileges.

The reasoning is that the real caller is also the one used in `PendingIntentRecord` to determine if the process is visible and the goal from a security perspective is that a process cannot grant a BAL token to itself.

Tests already exist in `BackgroundActivityLaunchTest` when the `balCheckBroadcastWhenDispatched` flag is enabled.

Test: atest BackgroundActivityLaunchTest
Flag: EXEMPT bugfix
Bug: 421197489
Bug: 422788436
Change-Id: I659d2fade206ea8435558f6cb3c626a932e76679