Stop relying on PACKAGE_VERIFIER_ENABLE.

This setting is being removed, and it no longer controls whether a
package verifier will be invoked. (Each verifier may have its own
controls, but they are outside the framework.)

Test: atest VerifyAppsOverUsbPreferenceControllerTest
Bug: 135751498
Change-Id: I78029c3529f331a88fde42a7aac5c155954f825e