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