add setPreserveSrcDepth to replace PrefTable API for android

src/images/SkImageDecoder.cpp

Index: src/images/SkImageDecoder.cpp
diff --git a/src/images/SkImageDecoder.cpp b/src/images/SkImageDecoder.cpp
index fe61906357d43bd88bb52299dbeb903a4d60f90e..8447ed46f71abc0e39492e5aeb47a6c614257da9 100644
--- a/src/images/SkImageDecoder.cpp
+++ b/src/images/SkImageDecoder.cpp
@@ -22,6 +22,7 @@ SkImageDecoder::SkImageDecoder()
     , fAllocator(NULL)
     , fSampleSize(1)
     , fDefaultPref(kUnknown_SkColorType)
+    , fPreserveSrcDepth(false)
     , fDitherImage(true)
 #ifdef SK_SUPPORT_LEGACY_BITMAP_CONFIG
     , fUsePrefTable(false)
@@ -156,7 +157,6 @@ void SkImageDecoder::setPrefConfigTable(const PrefConfigTable& prefTable) {
 SkColorType SkImageDecoder::getPrefColorType(SrcDepth srcDepth, bool srcHasAlpha) const {
     SkColorType ct = fDefaultPref;
 #ifdef SK_SUPPORT_LEGACY_BITMAP_CONFIG
-
     if (fUsePrefTable) {
         // Until we kill or change the PrefTable, we have to go into Config land for a moment.
         SkBitmap::Config config = SkBitmap::kNo_Config;
@@ -177,6 +177,19 @@ SkColorType SkImageDecoder::getPrefColorType(SrcDepth srcDepth, bool srcHasAlpha
         ct = SkBitmapConfigToColorType(config);
     }
 #endif
+    if (fPreserveSrcDepth) {
+        switch (srcDepth) {
+            case kIndex_SrcDepth:
+                ct = kIndex_8_SkColorType;
+                break;
+            case k8BitGray_SrcDepth:
+                ct = kN32_SkColorType;
+                break;
+            case k32Bit_SrcDepth:
+                ct = kN32_SkColorType;
+                break;
+        }
+    }
     return ct;
 }