Android: Better identify thirdparty C/C++ code

- The `cpu-features.{c,h}` code was only used by chance by the webm
  (libvpx) code, so I moved it there. It was actually introduced before
  that and wasn't in use, and libvpx just happened to be able to
  compile thanks to it being bundled.
  It could potentially be compiled on the fly from the Android NDK, but
  since we plan to replace the webm module by a GDNative plugin in the
  near future, I went the bundling route.

- `ifaddrs_android.h` is already provided in the Android NDK as
  `ifaddrs.h`, same as on other Unixes. Yet we cannot use it until we
  up the min API level to 24, where `getifaddrs` is first defined.
  I moved the files to `thirdparty/misc` and synced them with upstream
  WebRTC (only indentation changes and removal of `static` qualifiers).

Also removes dropped thirdparty files from COPYRIGHT.txt after changes
in #24105 and #24145.

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