String16 is moveable (noexcept)

String16 is still in use by AIDL compiler. Because String16 is not
noexcept-move-constructible, the C++ compiler will complain when it is
used with non-copyable types (such as ParcelFileDescriptor).

For example, when vector<Foo> is resized, copy-ctor of Foo is called,
which is not available.

  parcelable Foo {
    String s;
    ParcelFileDescriptor[] pfds;
  }

By providing noexcept move-ctor for String16, vector<Foo> can be resized
with no problem.

Btw, copy from StaticString16 is specialized for efficiency and move
from StaticString16 don't need to be different from copy.

Bug: 192136980
Test: libutils_test
Change-Id: I13744a2ceebf5781c3ef7f3a04237a6750b0db0a
diff --git a/libutils/String16.cpp b/libutils/String16.cpp
index faf90c2..c42cada 100644
--- a/libutils/String16.cpp
+++ b/libutils/String16.cpp
@@ -96,6 +96,12 @@
     acquire();
 }
 
+String16::String16(String16&& o) noexcept
+    : mString(o.mString)
+{
+    o.mString = getEmptyString();
+}
+
 String16::String16(const String16& o, size_t len, size_t begin)
     : mString(getEmptyString())
 {
@@ -126,6 +132,13 @@
     release();
 }
 
+String16& String16::operator=(String16&& other) noexcept {
+    release();
+    mString = other.mString;
+    other.mString = getEmptyString();
+    return *this;
+}
+
 size_t String16::size() const
 {
     if (isStaticString()) {