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
diff --git a/libs/utils/String16.cpp b/libs/utils/String16.cpp
index c856ceb..b09b728 100644
--- a/libs/utils/String16.cpp
+++ b/libs/utils/String16.cpp
@@ -93,6 +93,19 @@
 {
 }
 
+String16::String16(StaticLinkage)
+    : mString(0)
+{
+    // this constructor is used when we can't rely on the static-initializers
+    // having run. In this case we always allocate an empty string. It's less
+    // efficient than using getEmptyString(), but we assume it's uncommon.
+
+    char16_t* data = static_cast<char16_t*>(
+            SharedBuffer::alloc(sizeof(char16_t))->data());
+    data[0] = 0;
+    mString = data;
+}
+
 String16::String16(const String16& o)
     : mString(o.mString)
 {