Treat APEX installs as upgrades

It's not allowed to install a completely new APEX, hence APEX
installations should be treated as upgrades.

This change allows holders of INSTALL_PACKAGE_UPDATES permission to
silently install APEX upgrades.

Bug: 193085724
Bug: 190802380
Test: atest StagedInstallInternalTest
Change-Id: Idde7644122eeae6c444e3520f66ab1546ba7fdf6