Demote BinderGetCallingInMainThread from an error to a warning

Lint was updated, and adds this new error-level check that we
have preexisting occurances of.

Bug: 247885568
Bug: 249337906
Bug: 249335143
Test: m lint-check on internal master
Change-Id: I88c337dd1e12b2722a97e878b5277201df0c163d