Add the `setOnClickListener` for the `mAddDesktopButton`,
it will call the `createDesktop` API inside SystemUiProxy
to create a new Desk inside the display that the button
resides in.
Flag: com.android.window.flags.enable_multiple_desktops_frontend
com.android.window.flags.enable_multiple_desktops_backend
Fix: 391917345
Test: Manual, make sure the callback function `createDesk` is called
after clicking the button. No real desk will be created
for now, as the backend hasn't been implemented yet.
Change-Id: I70417a1ce0963b9b3c109c3f3dffaab808212d23
minch
d00382a09d