Move ArtField to native

Add linear alloc. Moved ArtField to be native object. Changed image
writer to put ArtFields after the mirror section.

Savings:
2MB on low ram devices
4MB on normal devices

Total PSS measurements before (normal N5, 95s after shell start):
Image size: 7729152 bytes
23112 kB: .NonMoving
23212 kB: .NonMoving
22868 kB: .NonMoving
23072 kB: .NonMoving
22836 kB: .NonMoving
19618 kB: .Zygote
19850 kB: .Zygote
19623 kB: .Zygote
19924 kB: .Zygote
19612 kB: .Zygote
Avg: 42745.4 kB

After:
Image size: 7462912 bytes
17440 kB: .NonMoving
16776 kB: .NonMoving
16804 kB: .NonMoving
17812 kB: .NonMoving
16820 kB: .NonMoving
18788 kB: .Zygote
18856 kB: .Zygote
19064 kB: .Zygote
18841 kB: .Zygote
18629 kB: .Zygote
3499 kB: .LinearAlloc
3408 kB: .LinearAlloc
3424 kB: .LinearAlloc
3600 kB: .LinearAlloc
3436 kB: .LinearAlloc
Avg: 39439.4 kB

No reflection performance changes.

Bug: 19264997
Bug: 17643507

Change-Id: I10c73a37913332080aeb978c7c94713bdfe4fe1c
diff --git a/runtime/utils_test.cc b/runtime/utils_test.cc
index 833427e..623a3ec 100644
--- a/runtime/utils_test.cc
+++ b/runtime/utils_test.cc
@@ -146,7 +146,7 @@
   mirror::Class* java_lang_String = class_linker_->FindSystemClass(soa.Self(),
                                                                    "Ljava/lang/String;");
 
-  mirror::ArtField* f;
+  ArtField* f;
   f = java_lang_String->FindDeclaredInstanceField("count", "I");
   EXPECT_EQ("int java.lang.String.count", PrettyField(f));
   EXPECT_EQ("java.lang.String.count", PrettyField(f, false));