CertPinInstallReceiver: remove files at boot time

In commit 60fb1aa9, CertPinInstallReceiver was updated to be
disabled when the platform flag was enabled. This prevents fresh devices
from ever installing the pins file. However, the onReceive is not called
unless the version available on the server is different from the current
version (as recorded by ConfigUpdater). Devices upgrading would still
have a lingering version of the pins. Add a clean up stage, ran at boot
time, to remove the files previously installed.

Test: Fresh install with flag disabled; check that pins file is available.
      Enable platform flag and reboot. The pins file and its metadata are gone.
Bug: 391205997
Flag: com.android.server.flags.certpininstaller_removal
Change-Id: I502369e21b5b66a1e413cac0298b0072a904bb2e