Fix null deref & update compress

Gainmap ctor always sets the native bitmap properly,
avoids potential confusion over mGainmapContents vs.
native bitmap pointer

Also call the fancier newer compress function

Test: SilkFX
Bug: 266628247
Change-Id: I89728a66202b4cdad54e8f163bc04dc419b8187b

graphics/java/android/graphics/Gainmap.java

@@ -96,8 +96,8 @@ public final class Gainmap implements Parcelable {
             throw new RuntimeException("internal error: native gainmap is 0");
         }
 
-        mGainmapContents = gainmapContents;
         mNativePtr = nativeGainmap;
+        setGainmapContents(gainmapContents);
 
         NoImagePreloadHolder.sRegistry.registerNativeAllocation(this, nativeGainmap);
     }

libs/hwui/hwui/Bitmap.cpp

@@ -464,7 +464,7 @@ bool Bitmap::compress(JavaCompressFormat format, int32_t quality, SkWStream* str
         SkBitmap baseBitmap = getSkBitmap();
         SkBitmap gainmapBitmap = gainmap()->bitmap->getSkBitmap();
         SkJpegEncoder::Options options{.fQuality = quality};
-        return SkJpegGainmapEncoder::EncodeJpegR(stream, baseBitmap.pixmap(), options,
+        return SkJpegGainmapEncoder::EncodeHDRGM(stream, baseBitmap.pixmap(), options,
                                                  gainmapBitmap.pixmap(), options, gainmap()->info);
     }
 #endif