Intoduce ResEntryWriter unit.

ResEntryWriter is responsible to write pairs of entry+value into binary
buffers and allows easy customization how and where entries are written.

This is no-op refactoring and all existing tests should pass.

Bug: b/249793372
Test: Existing tests
Change-Id: Ica98a25aeb49cc96cad28547c462b36316a8adb6
diff --git a/tools/aapt2/Android.bp b/tools/aapt2/Android.bp
index 7ddbe95..aa337e5 100644
--- a/tools/aapt2/Android.bp
+++ b/tools/aapt2/Android.bp
@@ -105,6 +105,7 @@
         "format/Container.cpp",
         "format/binary/BinaryResourceParser.cpp",
         "format/binary/ResChunkPullParser.cpp",
+        "format/binary/ResEntryWriter.cpp",
         "format/binary/TableFlattener.cpp",
         "format/binary/XmlFlattener.cpp",
         "format/proto/ProtoDeserialize.cpp",