Add a rule for Incompatible equals comparison

EqualsIncompatibleType checks is for equality test between
objects with incompatibles tpyes will always returns false.

Test: make google-tradefed-all javac-check
RUN_ERROR_PRONE=true
./tests/run_tradefed_presubmit.sh
Bug: 37854485

Change-Id: I4dc2aa122f6fa1da70dd409396d8d86674286d3f