Add HiddenAPIScope to replace use of SdkKind

Previously, the hidden API processing used SdkKind to identify the API
scopes, e.g. public, system, etc. that are of interest for hidden API
processing. Unfortunately, there is a mismatch between the SdkKind and
what hidden API processing needs. e.g. SdkKind includes values that are
not used by hidden API processing and hidden API processing needs
additional API scope specific information not provided by SdkKind. The
apiScope struct used in sdk_library.go is also not a suitable
representation for similar reasons.

This change adds the HiddenAPIScope (following a similar approach as
apiScope) that rectifies that and uses it as a replacement for SdkKind
in most parts of the hidden API processing. The SdkKind is still used
for retrieving information from java_sdk_library[_import] modules.

Follow up changes will extend the HiddenAPIScope with more information.

Bug: 179354495
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
      - make sure that this change has no effect on the generated flags.
Change-Id: I97968f58535121652852b8d25217aa288afd2bfd