Set PRIVATE_PREFIX when using $(transform-prebuilt-to-target)

If PRIVATE_PREFIX is unset transform-prebuilt-to-target references
$(DISPLAY) instead of $(HOST_DISPLAY) or $(TARGET_DISPLAY), which
causes unnecessary kati regenerations when the DISPLAY environment
variable changes.

Bug: 36901093
Bug: 37333696
Test: grep for $DISPLAY in out/build-${TARGET_PRODUCT}.ninja
Change-Id: Ia9802fc2b07da9b74e8f80dfdcef121408cd9817
(cherry picked from commit d2e778fb)