AAPT2: Fix JavaDoc first sentence extraction.

The old algorithm for detecting the first sentence of a JavaDoc comment
looked for the first occurence of '.'. This does not work when code or a
{@link android.R.styleable} link is encountered in the first sentence.

Switch to checking for whitespace characters after the '.' character.

Bug: 62900335
Test: make aapt2_tests
Change-Id: I8238f6a6304c9c2f92e2e576ca8962a59c2b20ea
diff --git a/tools/aapt2/text/Utf8Iterator.h b/tools/aapt2/text/Utf8Iterator.h
index 6923957..9318401 100644
--- a/tools/aapt2/text/Utf8Iterator.h
+++ b/tools/aapt2/text/Utf8Iterator.h
@@ -29,6 +29,10 @@
 
   bool HasNext() const;
 
+  // Returns the current position of the iterator in bytes of the source UTF8 string.
+  // This position is the start of the codepoint returned by the next call to Next().
+  size_t Position() const;
+
   void Skip(int amount);
 
   char32_t Next();
@@ -39,6 +43,7 @@
   void DoNext();
 
   android::StringPiece str_;
+  size_t current_pos_;
   size_t next_pos_;
   char32_t current_codepoint_;
 };