Add String16::c_str and start using it.

This is for parity with String8::c_str and in general the first step
in transition from String8|16.string() - to make it more similar to
std::string.

Bug: 295394788
Test: mma -j
Change-Id: I6c1411bef07c761fb2fb3fb38c27801ac4cffc57
diff --git a/libutils/String16.cpp b/libutils/String16.cpp
index 68642d8..38d483e 100644
--- a/libutils/String16.cpp
+++ b/libutils/String16.cpp
@@ -26,7 +26,7 @@
 
 static const StaticString16 emptyString(u"");
 static inline char16_t* getEmptyString() {
-    return const_cast<char16_t*>(emptyString.string());
+    return const_cast<char16_t*>(emptyString.c_str());
 }
 
 // ---------------------------------------------------------------------------
@@ -112,10 +112,7 @@
 
 String16::String16(const char16_t* o, size_t len) : mString(allocFromUTF16(o, len)) {}
 
-String16::String16(const String8& o)
-    : mString(allocFromUTF8(o.string(), o.size()))
-{
-}
+String16::String16(const String8& o) : mString(allocFromUTF8(o.c_str(), o.size())) {}
 
 String16::String16(const char* o)
     : mString(allocFromUTF8(o, strlen(o)))
@@ -173,7 +170,7 @@
         LOG_ALWAYS_FATAL("Not implemented");
     }
 
-    return setTo(other.string()+begin, len);
+    return setTo(other.c_str() + begin, len);
 }
 
 status_t String16::setTo(const char16_t* other)
@@ -200,7 +197,7 @@
 }
 
 status_t String16::append(const String16& other) {
-    return append(other.string(), other.size());
+    return append(other.c_str(), other.size());
 }
 
 status_t String16::append(const char16_t* chrs, size_t otherLen) {
@@ -286,7 +283,7 @@
 {
     const size_t ps = prefix.size();
     if (ps > size()) return false;
-    return strzcmp16(mString, ps, prefix.string(), ps) == 0;
+    return strzcmp16(mString, ps, prefix.c_str(), ps) == 0;
 }
 
 bool String16::startsWith(const char16_t* prefix) const