Don't use readback hardware if hdr layers need tonemapping

But, if we want to preserve display colors, we should still use readback
hardware

Bug: 431746508
Flag: com.android.graphics.surfaceflinger.flags.readback_screenshot
Test: adb screencap with hdr on-screen
Change-Id: I8a12ebb2d2c14034e94579812fd6392dc3fed584