Fix errors to combine with previous declaration specifier

Summary:
Fixes the following:
system/core/include/utils/Unicode.h:25:18: error: cannot combine with previous 'type-name' declaration specifier
typedef uint32_t char32_t;
                 ^
system/core/include/utils/Unicode.h:26:18: error: cannot combine with previous 'type-name' declaration specifier
typedef uint16_t char16_t;
                 ^

system/core/include/cutils/jstring.h:27:18: error: cannot combine with previous 'type-name' declaration specifier
typedef uint16_t char16_t;
                 ^

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D119