Fix the xml parsing of multi-package sessions.

The existing code didn't restore the child sessions, since the
XmlPullParser object was advanced within the
PackageInstallerSession#readFromXml code (in order to read grant
permissions).

Slightly refactor the code to make it more self-contained and testable,
and adding a test to make sure that the most important bits of a
session, or of a group of sessions, are dumped and restored properly.

Bug: 118865310
Bug: 109941548
Test: atest FrameworksServicesTests:PackageInstallerSessionTest;
verified that install_sessions.xml is correctly written and restored in
presence of multi-group sessions.
Change-Id: I5aef61f64e7223844f11661ee068dd3c1e112611