Export timestamp of the creation of install session. The timestamp is an

useful signal for downstream to determine post-creation operation.

One specific case is in Launcher, we update the homescreen as a response
of session creation. Since launcher needs to register a listener, it
misses some of the sessions which were created while the Launcher process
was dead. This API allows launcher to keep a track of
"last-created-session" and on the next process start, go through all the
sessions to determine the new sessions.

The main use case would be for restore in Launcher, where if it sees
new restore sessions which were previously not there, Launcher will
"undelete" some of the restored app icons and widgets.

Bug: 141472083
Test: PackageInstallerSessionTest
Change-Id: I52d18c5bb8e687f11bb27169146d332f45699ede