Allow java_host_binary to be used with genrule.

This is done by implementing the HostToolPath interface for java
Binary objects.

Currently, in order for a java_binary_host to be used in this way the
java_host_binary must include:
    compile_multilib: "first"

Otherwise the genrule is unable to find the correct variant.

Test: ./build_test.bash
Test: Manual

Bug: 68397812

Change-Id: I6849488dc13fa8c383df69d00f62ad815ec17876

java/java.go

@@ -868,6 +868,10 @@ type Binary struct {
 	binaryFile  android.OutputPath
 }
 
+func (j *Binary) HostToolPath() android.OptionalPath {
+	return android.OptionalPathForPath(j.binaryFile)
+}
+
 func (j *Binary) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 	j.Library.GenerateAndroidBuildActions(ctx)