Speed up non-AB ota_from_target_files in the build system

We've just created the target files package and still have the extracted
directory, so pass that to ota_from_target_files so that it doesn't have
to re-extract what we just compressed.

This saves a little bit of time -- for bullhead-userdebug on internal
master, this brings the time to build an OTA from ~340s -> ~310s. Much
of the time is still spent generating and signing the OTA.

Test: lunch bullhead-userdebug; m -j otapackage
Test: bullhead-ota-*.zip is identical before/after
Change-Id: Ib51cf6b02123e9c49280e9957c6590645470a52c