[ADI][63/N] support mutli-package installation

Enables developer verification for multi-package sessions by verifying child sessions and merging results.

This is done by the following changes:

1. Refactors DeveloperVerifierCallback to use CompletableFuture with a new DeveloperVerificationFutureResult class to asynchronously report outcomes, user actions requirements, and failures.

2. DeveloperVerification results from multiple child sesions are merged
   into one. We will handle the user action requirements in a followup
change.

FLAG: android.content.pm.verification_service
BUG: 360129657
Test: atest CtsPackageInstallerCUJDeveloperVerificationTestCases
Test: atest com.android.server.pm.verify.developer.DeveloperVerifierControllerTest
Test: atest CtsPackageInstallSessionTestCases android.packageinstaller.install.cts.SessionTest#confirmMultiPackageInstallation
Test: manually with "set-developer-verification-result" and "install-multi-package"

Change-Id: Ideca3db0160a93676e6d0985f9fb8b7d28a69a03