[ADI][64/N] support user actions in multi-package installs

+ Support user actions in multi-package by preserving the
  CompletableFuture as part of the session state
+ Moved emergency bypass check back to per-child-session level
+ Improved merging logic by combining extension response and error
  messages from multiple child sessions
+ Code cleanup

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

Change-Id: Idfb9925075cc3ca085af0a68ba294d45ad2a66d7