Don't allow shell to spoof initiating package name.

This preserves the ability to override the installer package name
(installer of record). But it prevents overriding the initiating
package name (real installing package) which should always be null for
an adb installed package.

Bug: 160224791
Test: manual
Test: atest PackageManagerTests PackageVerifierTest
Change-Id: I586b524d11dab075034ca05a87fac819cb99b61b