Revert "Flag to control if generated SDK prebuilts are preferred."

This reverts commit d0b99cea.

Reason for revert: Not needed anymore. We will simple exclude the 
gernated Android.bp files from prebuilt drops instead, it keeps
things simpler.

Change-Id: I5b01b2e01197f9fceb4630c8ade95f0d1d0f33f4