Fix PDK build

This commit disables "bootstat" in PDK builds because "bootstat" depends
on "libstatslog" (from "frameworks/base") which is not included in PDK
builds as well.

Test: Build a target (described in http://b/72961456) with
`platform.zip` built from master FSK source tree.

Bug: 72961456
Change-Id: I06b1555694510e17ea82d5c6dfcdeaf99b905e4d