Clean up srcjar directories after each rule

The srcjar directories just confuse people looking for generated
files, clean them up after each rule.

Test: m checkbuild
Change-Id: If712ce7d4922619fd8420ae8f89830fe7824114d

java/builder.go

@@ -47,7 +47,8 @@ var (
 				`$processorpath $processor $javacFlags $bootClasspath $classpath ` +
 				`-source $javaVersion -target $javaVersion ` +
 				`-d $outDir -s $annoDir @$out.rsp @$srcJarDir/list ; fi ) && ` +
-				`${config.SoongZipCmd} -jar -o $out -C $outDir -D $outDir`,
+				`${config.SoongZipCmd} -jar -o $out -C $outDir -D $outDir && ` +
+				`rm -rf "$srcJarDir"`,
 			CommandDeps: []string{
 				"${config.JavacCmd}",
 				"${config.SoongZipCmd}",