Add PRODUCT_FULL_TREBLE.

PRODUCT_FULL_TREBLE: whether the device has Treble fully enabled (no
passthrough hals, vendor.img, split sepolicy, etc..)

PRODUCT_FULL_TREBLE_OVERRIDE: used if someone wants to consider a device
as full Treble even if it didn't launch with Android O.

Bug: 35809750
Test: PRODUCT_FULL_TREBLE set appropriately with
PRODUCT_FULL_TREBLE_OVERRIDE and PRODUCT_SHIPPING_API_LEVEL = 24/25

Change-Id: I069448beaf26c70acb6c45d4228d8fdfe9476410