Add a unit test for receiving 'finish' message after channel close

After the input channel is closed, the messages that were previously
written to the fd should still be readable. However, in some cases, if
the fd is closed with data in the fd's buffer, the data may be
discarded. This is confirmed by the attached unit test.

Add an explanation of this to the InputChannel implementation, so that
this behaviour is documented.

Bug: 376713684
Test: TEST=libinput_tests; m $TEST && adb sync data && adb shell -t /data/nativetest64/$TEST/$TEST --gtest_filter="*ReceiveAfterClose*" --gtest_repeat=100000 --gtest_break_on_failure
Test: TEST=libinput_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST --gtest_filter="*ReceiveAfterClose*" --gtest_repeat=100000 --gtest_break_on_failure
Flag: TEST_ONLY
Change-Id: I67a6a6432c4756283c8f2cca3c01210a3bcdb42e