Property api_srcs added for api-only source files

New property api_srcs is added to java_sdk_library. The property is used
to specify set of source files that will be part of the API but not part
of the runtime library. Currently, this is only for the legacy http
library (org.apache.http.legacy) where some framework classes (such as
android.net.SslError which are compiled into framework.jar) are
registered as its API for historical reason.

Bug: 77575606
Test: m -j
Change-Id: I36d153f4815f8e4c939d5eeec632a38e7943b4b0