Add flag for degrading HDR to SDR during mirroring

Bug: 443315364
Flag: com.android.graphics.surfaceflinger.flags.md_degrade_hdr
Test: builds
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:fa856fe8b5db5f2afaed07eef97cbbf608b9b20f
Merged-In: I4a5bc3d71d8c6fa69e86265a5720c6f3e0c4e100
Change-Id: I4a5bc3d71d8c6fa69e86265a5720c6f3e0c4e100