Generate wayland server protocol headers.

Added a build target to run wayland_scanner on the protocol xml files to
generate all the *-server-protocol.h files, rather than only the
*-client-protocol.h files as before.

Test: - Add 'generated_headers: ["wayland_extension_server_protocol_headers"]'
        to any other cc_binary or cc_library build target (T)
      - m T
      - Check that out/soong/.intermediates/external/wayland-protocols/\
        wayland_extension_server_protocol_headers/gen/*-server-protocol.h
	exists, and that source files in T can include them.

Change-Id: I940a3845c400bd05020295229a28d0202b6d4538