Don't export kMinCompressedSize.

kMinCompressedSize was defined as a global constant, so it
was exported by default and possibly causing duplicate
symbol issues for applications. In particular, libvpx also
exports this symbol.

Instead, move the definition into the only function which
references is. As a local variable it is hidden from the
linker and not part of the global symbol table.

Change-Id: If8b3bc4c487197237adb04749cd3dd4792e39893