Add sys config to handle apexes that are allowed to be updated

Unlike staged installer check, we can't check if given APEX package is
allowed to be updated at session creation time, since we don't have
knowledge of the package being installed yet. Instead, the check is
implemented in PackageInstallerSession#handleInstall.

Like staged install check, allowed apex update check has similar
exemptions (adb is allowed to update any APEX,
`adb shell pm --bypass-allowed-apex-update-check` makes next install
session bypass the check).

In order to implement these exemptions, a new
INSTALL_DISABLE_ALLOWED_APEX_UPDATE_CHECK flag that can only be set by
system is added. PackageInstallerSession will skip the APEX update
checks if INSTALL_DISABLE_ALLOWED_APEX_UPDATE_CHECK is set.

Bug: 189274479
Test: atest CtsStagedInstallHostTestCases
Test: atest GtsStagedInstallHostTestCases
Test: atest FrameworksServicesTests:SystemConfigTest
Change-Id: I22921a3ac4d43011b565733d7a7183e5cdb4fe80
Merged-In: I22921a3ac4d43011b565733d7a7183e5cdb4fe80
(cherry picked from commit aafaaec0)