soong: globally enable -Wunreachable-code-loop-increment

This warning only flags code that's otherwise pretty trivial to refactor
into something more readable. It also catches nasty bugs, so turning it
on seems like a good idea.

Bug: 150166387
Test: TreeHugger
Change-Id: I0e095d83ba4a2dcd81f680efd8ea3d2a6fe1a970