Build VSoCService with SDK 28

A device whose first_shipping_api_level is 28+, all apks in the vendor
partition should be built against SDK 28+. As the first shipping api
level of cuttlefish is recently updated, VSoCService being built with
SDK 26 is causing GTS failure. (b/129744020)

Switching to 28.

Bug: 129744020
Bug: 129902764
Test: m VSoCService
Change-Id: I7a83639c72864cd78ceed27ed519dc7f63ae1d7c