find7-common/twrp: set device properties independly of the target device

TWRP-builds are universal for find7s/find7a and unified/non-unified
storage layout. Set ro.product.device to find7/find7a in init and
ro.build.product always to find7. This should give a good balance of
precision and recall for asserts in older and newer install-scripts.

Change-Id: Ib8251fd2f582fbe236829c65950950886e311eeb