AAPT2: Convert from Modified UTF-8 ResStringPool

Since ResStringPools are encoded using Modified UTF-8, retrieving
strings from the string pool convert the strings to UTF-8 before
returning.

Bug: 114734350
Test: m aapt2_tests
Change-Id: Ib459018186f4c5b40f3f3786425a335ecfb9ed02
diff --git a/tools/aapt2/util/Util.h b/tools/aapt2/util/Util.h
index 36b7333..c6e8e6e 100644
--- a/tools/aapt2/util/Util.h
+++ b/tools/aapt2/util/Util.h
@@ -199,6 +199,7 @@
 
 // Converts a UTF8 string into Modified UTF8
 std::string Utf8ToModifiedUtf8(const std::string& utf8);
+std::string ModifiedUtf8ToUtf8(const std::string& modified_utf8);
 
 // Converts a UTF8 string to a UTF16 string.
 std::u16string Utf8ToUtf16(const android::StringPiece& utf8);