Ignore InstallSessions with null appPackageName.

This changelist filters out InstallSessions without a appPackageName
set. This prevents trying to converting the session another class that
requires the package name to be non-null.

Test: atest PackageInstallerMonitorTest#installSessions_ignoreNullPackageNameSessions
Flag: EXEMPT bugfix
Fixes: 362276271
Change-Id: I6109f00b9ffbcbdbd41087ba0e7f0cc0c39381e8