Modified functions to return pointers where appropriate

GetStringChars will return a pointer to the underlying char array iff
it is not movable. Otherwise, it will return a copy of the char array.
For consistency, the null terminating character has been removed as the
specification for a jchar strings are not null terminated:

http://developer.android.com/training/articles/perf-jni.html

GetStringCritical will now always return a pointer to the char array.
The char array is pinned and moving gc is disabled until the pointer is
released.

Change-Id: I19c8cbaecc1f3f723d80acec074fb8c5e2d489c3