Dumpsys support for quick affordances.

Should aid in debugging.

Bug: 257066487
Test: Manually verified that running:

adb shell dumpsys activity service com.android.systemui/.SystemUIService KeyguardQuickAffordances

Properly outputs the right text, for example:

    KeyguardQuickAffordances:
    ----------------------------------------------------------------------------
    Slots & selections:
        bottom_start: home (capacity = 1)
        bottom_end is empty (capacity = 1)
    Available affordances on device:
        home ("Home")
        wallet ("Wallet")
        qr_code_scanner ("QR code scanner")

Change-Id: I5278e977a775aa7346914514a964fdacfc082bf0