Link libinput statically for inputflinger_tests

Linking statically makes it easier to debug tests by letting developer
add logs to libinput and have that reflected in the 'atest' runs for
inputflinger_tests without having to resync.

Also in this CL, enable C++20 for libinput, which would allow us to do
things like specify the default comparison operator.

Bug: 167946763
Test: cd services/inputflinger && atest
Change-Id: Iaa3f2e34d79fa000ef42b74237efae6233b44674