Fix nullability annotations on equals methods

The equals method in Object takes a Nullable parameter, so all
subclasses must do the same. Fixing these issues allows re-enabling
the InvalidNullabilityOverride error, fix also catches more severe
issues.

Bug: 309908788
Test: m checkapi
Change-Id: I6b4040aee02b0f2838ba27e2edd35bdcdcbcfc8b