Create /data/ota_package for OTA packages.

We will store OTA packages there for both A/B and non-A/B OTAs. The
directory will be accessed by GMSCore (for both), uncrypt (non-A/B),
update_engine (A/B), update_verifier (A/B) and possibly system server
(for non-A/B OTAs to clean up half-way uncrypt'd packages).

Bug: 28944800
Change-Id: I5aa8156ec5052bd15dfadd4d8c28925d464e4401