add icon to flashlight slider

Disable slider when flashlight is not adjustable.

Bug: 423695631
Bug: 413736768

Flag: com.android.systemui.flashlight_strength
Test: atest FlashlightSliderViewModelTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c0bba90f150bef964a487d7d163e63a7287859ea)
Merged-In: Ia615e8bb6e8607f03ac743c363c4410070144870
Change-Id: Ia615e8bb6e8607f03ac743c363c4410070144870