aidl: put @utf8InCpp

String type in AIDL is mapped to String16 in C++, which doesn't work
well with ParcelFileDescriptor. (String16 is not moveable while PFD is
moveable but not copyable.)

Putting @utf8InCpp makes String type in AIDL std::string in C++.
std::string has no such problems.

Besides, there's no good reason to use String16 in C++.

Bug: 192136980
Test: MicrodroidHostTestCases, VirtualizationTestCases
Change-Id: I47ffd5f9083a7bd446fbff6a806f1d31295e6cd3
4 files changed