Add android::base::expected

android::base::expected is an Android implementation of the
std::expected proposal.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0323r7.html

For usage, refer to the expected.h header file and
expected_test.cpp

Bug: 132145659
Test: libbase_test

Change-Id: I65d3a1ecf8654d9858989755dfd0065c81f7b209