Add error-prone support

Add support for compiling java sources with the error-prone tool.

Test: m -j checkbuild
Change-Id: Ieb4ee0e05f8f34a52ed7bcf1c7cbacf1c9c4d0b5

android/package_ctx.go

@@ -16,6 +16,7 @@ package android
 
 import (
 	"fmt"
+	"strings"
 
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/pathtools"
@@ -75,6 +76,25 @@ func (p AndroidPackageContext) SourcePathVariable(name, path string) blueprint.V
 	})
 }
 
+// SourcePathsVariable returns a Variable whose value is the source directory
+// appended with the supplied paths, joined with separator. It may only be
+// called during a Go package's initialization - either from the init()
+// function or as part of a package-scoped variable's initialization.
+func (p AndroidPackageContext) SourcePathsVariable(name, separator string, paths ...string) blueprint.Variable {
+	return p.VariableFunc(name, func(config interface{}) (string, error) {
+		ctx := &configErrorWrapper{p, config.(Config), []error{}}
+		var ret []string
+		for _, path := range paths {
+			p := safePathForSource(ctx, path)
+			if len(ctx.errors) > 0 {
+				return "", ctx.errors[0]
+			}
+			ret = append(ret, p.String())
+		}
+		return strings.Join(ret, separator), nil
+	})
+}
+
 // SourcePathVariableWithEnvOverride returns a Variable whose value is the source directory
 // appended with the supplied path, or the value of the given environment variable if it is set.
 // The environment variable is not required to point to a path inside the source tree.