Add LOCAL_SDK_VERSION in platform_library

The platform_library is using only public API. However it is defined
java:platform which can use internal API, since it is not
defined LOCAL_SDK_VERSION.
So, LOCAL_SDK_VERSION := current is added in platform_library, because
it can be turn into errors when link type check becomes more strict.

Bug: 69899800
Test: mma -j under PlatformLibrary dir
Change-Id: I007da51ed50236b8f1de331afabc46906c8b47aa