Introduce vendor.mime.types

Like mime.types and android.mime.types, this file specifies mappings
between MIME types and file extensions. Unlike those files, it can
only be used to define _additional_ mapping but not modify (change,
remove) any mappings defined by those files.

This is done by prepending '?' to every line element from
vendor.mime.types that doesn't already have one; when there is a
leading "?", it is ignored so that it's okay to move a line from
{android,vendor}.mime.types without necessarily changing it.

Test: Checked manually that vendor.mime.types works as expected.
  Specifically, after adding these lines to vendor.mime.type:

    audio/mpeg testmpeg
    audio/testmpeg mp3
    ?mime/foo ?fooext

  the following test passes:

    MimeTypeMap map = MimeTypeMap.getSingleton();
    // Original mapping is unchanged
    assertEquals("mp3", map.getExtensionFromMimeType("audio/mpeg"));
    assertEquals("audio/mpeg", map.getMimeTypeFromExtension("mp3"));

    // Map from the key to existing value is added
    assertEquals("audio/mpeg", map.getMimeTypeFromExtension("testmpeg"));
    assertEquals("mp3", map.getExtensionFromMimeType("audio/testmpeg"));

    // Completely new mapping is added both ways
    assertEquals("mime/foo", map.getMimeTypeFromExtension("fooext"));
    assertEquals("fooext", map.getExtensionFromMimeType("mime/foo"));

Bug: 141842825
Change-Id: Iaf918ce39324709ff58a8e0f9612e4827a673323