new String8, String16 ctors to initialize empty static strings with static linkage

when libutils is statically linked, the ordering of the static
initializer is not guaranteed and therefore it's unsafe to use
empty static strings: e.g.:

static String8 sThisStaticStringIsNotSafe;

instead, this new constructor can be used:

static String8 sThisStaticStringIsSafe(kEmptyString);

Change-Id: Ia3daf1cab1c97d021c0ee9c2b394b5e27e8d6c0d