keys: avoid duplicating recovery keys

Setting PRODUCT_EXTRA_RECOVERY_KEYS duplicates the default key and
makes it so we can't set an extra key earlier in the build

Change-Id: I239d9994d9ad40b1779877ea2e89b55bd02e4c00
(cherry picked from commit ac8f2eb0472a4c4cf542e4a0a9b8f2c848174ccc)
Merge remote-tracking branch 'cm/stable/cm-11.0' into cm-11.0-tonyp