Fix unknown sources app install confirmation

Allow the package install confirmation dialog to appear properly after
the user turns on unknown sources. Also fix an incorrect session ID
of -1 being shown in related log warnings.

"Finish PIA if UnknownSourcesListener would like to start a PIA again"
is a change that caused this feature to fail for third-party app stores.
This change resolves that issue.

Issue: calyxos#2081
Test: atest CtsPackageInstallTestCases
Test: Manual: Install a third-party app store e.g. F-Droid. Try to
install an app. Tap Settings on the prompt to configure unknown sources.
Tap to turn on unknown sources. The install confirmation dialog appears.

BUG: 326612529
Change-Id: Idd6667cbd79945d849b74a62122162ae72d99210