Remove dependencies on Android's forked decoder libraries

gyp/libpng.gyp

 # Copyright 2014 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'skia_warnings_as_errors': 0,
   },
   'targets': [
     {
       'target_name': 'libpng',
       'type': 'none',
       'conditions': [
-        [ 'skia_os == "android"', {
-            'dependencies':              [ 'android_deps.gyp:png' ],
-            'export_dependent_settings': [ 'android_deps.gyp:png' ],
-        },{
-            'dependencies':              [ 'libpng.gyp:libpng_static' ],
-            'export_dependent_settings': [ 'libpng.gyp:libpng_static' ],
-        }]
-      ]
-    },
-    {
-      'target_name': 'libpng_static_when_possible',
-      'type': 'none',
-      'conditions': [
         [ 'skia_android_framework', {
             'dependencies':              [ 'android_deps.gyp:png' ],
             'export_dependent_settings': [ 'android_deps.gyp:png' ],
         },{
             'dependencies':              [ 'libpng.gyp:libpng_static' ],
             'export_dependent_settings': [ 'libpng.gyp:libpng_static' ],
         }]
       ]
     },
     {
@@ skipping 74 matching lines @@
       'conditions': [
         ['arm_neon_optional', {
           'cflags': [
             '-mfpu=neon',
           ],
         }],
       ],
     }
   ]
 }