Fix link-type check warnings for platformprotosnano

platformprotosnano is used by cts-amwm-util and this is causing
link-type check warning because cts-amwm-util is built using SDK but
platformprotosnano isn't.

Since platformprotosnano can actually be built with SDK APIs only,
setting LOCAL_SDK_VERSION to 'current' instead of building against
core-oj and core-libart which contain private APIs.

LOCAL_SDK_VERSION := core_current has been tried but it didn't work
because libprotobuf-java-nano is using Android APIs (thus marked as
java:sdk). java:core -> java:sdk dependency is not allowed.

Bug: 69899800
Test: m -j platformprotosnano
Change-Id: I14a9a80bb218dae63e8320b7aab40d7ad48a7a13