msm8974: Clean up prebuilts

 * Add correct files for Widevine L3
 * Disable PPPreference, it can't be used with partial update
 * Remove secure display, we don't support it
 * Purge deprecated board flags

Change-Id: I42f016ac52db47777a3feeb4aa5e7170877bd93e
(cherry picked from commit dd69e76c46909517b6ab64e7a08d372455853d73)