Add ForwardingOs to CorePlatformApi.

The class was already annotated @CorePlatformApi, but that had
no effect because ForwardingOs.java was in non_openjdk_java_files.
This CL moves it to non_openjdk_javadoc_files and marks it @hide,
which has the effect of adding it to CorePlatformApi.

Test: Manually checked that frameworks code referring to ForwardingOs
      does now compile.
Bug: 118100586

Change-Id: I2e0d88fec15cf20e119e1d1bffed98b5937babf1
2 files changed