Prevent arbitrary installers installing apps as instant

To this end, we enforce that only system installer could create the
install session when the install as instant flag is set.

Bug: 74401586
Test: atest SessionTest#confirmInstantInstallationFails
Test: atest InstallSessionTransferTest
Test: atest CtsSignedConfigHostTestCases
Test: Play could install instant apps
Test: adb install --instant works
Change-Id: Icb8088a5773a7b47ebfe47b7630e7b89f01a9563