Use UTF-8 strings to avoid duplicate caching, part 1

StringBlock instances containing UTF-8 strings use a cache to convert
into UTF-16, but using that cache and then using a JNI call to NewString
causes the UTF-8 string as well as two copies of the UTF-16 string to
be held in memory. Getting the UTF-8 string directly from the StringPool
eliminates one copy of the UTF-16 string being held in memory.

This is part 1. Part 2 will include ResXMLParser optimizations.

Change-Id: Ibd4509a485db746d59cd4b9501f544877139276c