Add support for LOCAL_PRIVATE_PLATFORM_APIS.

The only enforcement right now is that modules don't specify both
LOCAL_SDK_VERSION and LOCAL_PRIVATE_PLATFORM_APIS, but a warning
can be turned on by setting the JAVA_SDK_ENFORCEMENT_WARNING variable.

Bug: 73535841
Test: make w/ and wo/ flag.
Change-Id: I3cc720bf060e2bc789e10884ff888d527b91cb50