Add gainmap support for screencap

There still would need to be support added to the PNG spec to properly
support gainmaps without affecting downstream clients. So, this patch:

1. Allows for screencap to (temporarily) export jpegs
2. Adds plumbing in HWUI's apex layer to encode gainmaps
3. Wires up the attachGainmap flag to allow screenshots to output a
   gainmap

Bug: 329470026
Flag: com.android.graphics.surfaceflinger.flags.true_hdr_screenshots
Test: adb screencap -j sdcard/test.jpeg
Change-Id: I210a3e24ad2cfd6e0c0a954f42b9171d9e82e991