Convert coverageMutator to a TransitionMutator

Convert coverageMutator to a TransitionMutator as a step towards
variants-on-demand.

Bug: 319288033
Test: coverage_test.go
Test: treehugger coverage builds
Change-Id: Ic50c0040dea8b42c36b5d784221daa00b7b0d379
diff --git a/rust/rust.go b/rust/rust.go
index 34ce4c5..245ed2e 100644
--- a/rust/rust.go
+++ b/rust/rust.go
@@ -535,7 +535,7 @@
 
 var _ cc.Coverage = (*Module)(nil)
 
-func (mod *Module) IsNativeCoverageNeeded(ctx android.BaseModuleContext) bool {
+func (mod *Module) IsNativeCoverageNeeded(ctx android.IncomingTransitionContext) bool {
 	return mod.coverage != nil && mod.coverage.Properties.NeedCoverageVariant
 }