Update AImageDecoder_getAlphaFlags return value

Bug: 135133301
Test: I2ea58bede5cf49a7947c96e7e6e37f967632ad6b

BAD_PARAMETER is a more sensible value than -1. The other return will
not be reached, but the code is clearer.

Change-Id: I4bee8f238f61a6c6cd13b5458f201f34d34315d9
diff --git a/native/graphics/jni/imagedecoder.cpp b/native/graphics/jni/imagedecoder.cpp
index 2ef203d..a13cc02 100644
--- a/native/graphics/jni/imagedecoder.cpp
+++ b/native/graphics/jni/imagedecoder.cpp
@@ -225,13 +225,12 @@
 
 int AImageDecoderHeaderInfo_getAlphaFlags(const AImageDecoderHeaderInfo* info) {
     if (!info) {
-        // FIXME: Better invalid?
-        return -1;
+        return ANDROID_IMAGE_DECODER_BAD_PARAMETER;
     }
     switch (toDecoder(info)->mCodec->getInfo().alphaType()) {
         case kUnknown_SkAlphaType:
             LOG_ALWAYS_FATAL("Invalid alpha type");
-            return -1;
+            return ANDROID_IMAGE_DECODER_INTERNAL_ERROR;
         case kUnpremul_SkAlphaType:
             // fall through. premul is the default.
         case kPremul_SkAlphaType: