Do not disable prebuilts for sdk builds

The artifacts in the SDK zip should be OK to build with a prebuilt
stubs enabled, even if prebuilt apexes are not, because the system
images in the SDK stub are not currently used.

Bug: 205008975
Bug: 204407658
Test: m sdk
Change-Id: Ia78cf87743ff394dcfaa1b43556b230c0dda8742