Use unordered_map for connections

Currently, keyedvector is used for keeping track of connections by file
descriptors. Instead, use supported api std::unordered_map.
Also, no longer remove raw indices into the map, because that's
implementation detail of map that should not be user-facing.

Bug: 70668286
Test: SANITIZE_TARGET=hwaddress atest libinput_tests inputflinger_tests
Change-Id: Id31b0002a7ff2310dcc506f834722b1e69a05571