Raise progaurd library to platform libraries even for "current" SDK.

Without this change, if an app with "LOCAL_SDK_VERSION := current" links
against a static library that uses removed APIs, the app will fail
ProGuard processing because the removed APIs can't be found in the
current SDK. This works around the issue.

Bug: 18510449
Bug: 20667396
Change-Id: Ia6e2a81cf5ecba972d155e56238ea946f8539411