PackageManager: Limit the platform-key protection to test-keys builds

This block (change I7390b3bdf5289411f7a335c69635bed9f74bbfb3) is intended
to protect the system from abuse of well-known keys. That's not the case
if the system is signed with dev or release keys.

Change-Id: Ifb66f775dd28c577447634ae177442732a18b592
Merge "PackageManager: Limit the platform-key protection to test-keys builds" into cm-10.2
Merge branch 'cm-10.2' of git://github.com/CyanogenMod/android_frameworks_base into cm-10.2
Allow homescreen widgets on the lockscreen (1/2)
Change-Id: I8ecdd0e3f3ec008435bc637373ec923422211da9