Support per-layer color transforms when local tonemapping

* Preserve alpha channel when tonemapping
* Don't tonemap when a layer is entirely SDR

Bug: 329464641
Flag: com.android.graphics.surfaceflinger.flags.local_tonemap_screenshots
Test: adb shell cmd color_display set-layer-saturation 0 com.google.android.apps.wellbeing com.google.android.youtube
Change-Id: I363475caac5af83f2ef30bc073cf8c5aa6e313d4