AudioMixingRule: more javadoc on mix role

        /**

         * Sets target mix role of the mixing rule.

         *

         * <p>The mix role indicates playback streams will be captured or recording source will be

         * injected. If not specified, the mix role will be decided automatically when

         * As each mixing rule is intended to be associated with an {@link AudioMix},

         * explicitly setting the role of a mixing rule allows this {@link Builder} to

         * verify validity of the mixing rules to be validated.<br>

         * The mix role allows distinguishing between:

         * <ul>

         * <li>audio framework mixers that will mix / sample-rate convert / reformat the audio

         *     signal of any audio player (e.g. a {@link android.media.MediaPlayer}) that matches

         *     the selection rules defined in the object being built. Use

         *     {@link AudioMixingRule#MIX_ROLE_PLAYERS} for such an {@code AudioMixingRule}</li>

         * <li>audio framework mixers that will be used as the injection point (after sample-rate

         *     conversion and reformatting of the audio signal) into any audio recorder (e.g. a

         *     {@link android.media.AudioRecord}) that matches the selection rule defined in the

         *     object being built. Use {@link AudioMixingRule#MIX_ROLE_INJECTOR} for such an

         *     {@code AudioMixingRule}.</li>

         * </ul>

         * <p>If not specified, the mix role will be decided automatically when

         * {@link #addRule(AudioAttributes, int)} or {@link #addMixRule(int, Object)} be called.

         *

         * @param mixRole integer value of {@link #MIX_ROLE_PLAYERS} or {@link #MIX_ROLE_INJECTOR}