Improved javadoc on UserVisibilityMediator.

/**

 * Class responsible for deciding whether a user is visible (or visible for a given display).

 *

 * <p>Currently, it has 2 "modes" (set on constructor), which defines the class behavior (i.e, the

 * <p>Currently, it has 3 "modes" (set on constructor), which defines the class behavior (i.e, the

 * logic that dictates the result of methods such as {@link #isUserVisible(int)} and

 * {@link #isUserVisible(int, int)}):

 *

 * <ul>

 *   <li>default: this is the most common mode (used by phones, tablets, foldables, automotives with

 *   just cluster and driver displayes, etc...), where the logic is based solely on the current

 *   foreground user (and its started profiles)

 *   <li>{@code MUMD}: mode for "(concurrent) Multiple Users on Multiple Displays", which is used on

 *   automotives with passenger display. In this mode, users started in background on the secondary

 *   display are stored in map.

 *   <li>default (A.K.A {@code SUSD} - Single User on Single Display): this is the most common mode

 *   (used by phones, tablets, foldables, cars with just cluster and driver displays, etc.),

 *   where just the current foreground user and its profiles are visible; hence, most methods are

 *   optimized to just check for the current user / profile. This mode is unit tested by

 *   {@link com.android.server.pm.UserVisibilityMediatorSUSDTest} and CTS tested by

 *   {@link android.multiuser.cts.UserVisibilityTest}.

 *   <li>concurrent users (A.K.A. {@code MUMD} - Multiple Users on Multiple Displays): typically

 *   used on automotive builds where the car has additional displays for passengers, it allows users

 *   to be started in the background but visible on these displays; hence, it contains additional

 *   maps to account for the visibility state. This mode is unit tested by

 *   {@link com.android.server.pm.UserVisibilityMediatorMUMDTest} and CTS tested by

 *   {@link android.multiuser.cts.UserVisibilityTest}.

 *   <li>no driver (A.K.A. {@code MUPAND} - MUltiple PAssengers, No Driver): extension of the

 *   previous mode and typically used on automotive builds where the car has additional displays for

 *   passengers but uses a secondary Android system for the back passengers, so all "human" users

 *   are started in the background (and the current foreground user is the system user), hence the

 *   "no driver name". This mode is unit tested by

 *   {@link com.android.server.pm.UserVisibilityMediatorMUPANDTest} and CTS tested by

 *   {@link android.multiuser.cts.UserVisibilityVisibleBackgroundUsersOnDefaultDisplayTest}.

 * </ul>

 *

 * <p>When you make changes in this class, you should run at least the 3 unit tests and

 * {@link android.multiuser.cts.UserVisibilityTest} (which actually applies for all modes); for

 * example, by calling {@code atest UserVisibilityMediatorSUSDTest UserVisibilityMediatorMUMDTest

 * UserVisibilityMediatorMUPANDTest UserVisibilityTest}. Ideally, you should run the other 2 CTS

 * tests as well (you can emulate these modes using {@code adb} commands; their javadoc provides

 * instructions on how to do so).

 *

 * <p>This class is thread safe.

 */

public final class UserVisibilityMediator implements Dumpable {