Use operator<< with String16 rather than char16_t*

std::stringstream's operator<< overload for char16_t* is deleted
after upgrading libc++ because it surprisingly outputs an address
rather than string content. See wg21.link/p1423r3.

Bug: 175635923
Test: m MODULES-IN-frameworks-native-cmds-cmd
Change-Id: I90602eca166d5da347f7efba22f21bc1a5d38a11