Fix ndk build.

The NDK gen_stub_libs.py doesn't support introduced=P yet:

  Traceback (most recent call last):
    File "build/soong/cc/gen_stub_libs.py", line 439, in <module>
      main()
    File "build/soong/cc/gen_stub_libs.py", line 429, in main
      versions = SymbolFileParser(symbol_file, api_map).parse()
    File "build/soong/cc/gen_stub_libs.py", line 234, in parse
      versions.append(self.parse_version())
    File "build/soong/cc/gen_stub_libs.py", line 269, in parse_version
      symbols.append(self.parse_symbol())
    File "build/soong/cc/gen_stub_libs.py", line 286, in parse_symbol
      tags = decode_api_level_tags(tags, self.api_map)
    File "build/soong/cc/gen_stub_libs.py", line 76, in decode_api_level_tags
      raise ParseError('Unknown version name in tag: {}'.format(tag))
  __main__.ParseError: Unknown version name in tag: introduced=P

Bug: N/A
Test: builds
Change-Id: Ifd91e525b30f3aa9ad8c889c9564cd769a3a603a