Re-enable prebuilt_test

Use a temporary directory as the build directory during tests so files
don't get written to the source tree.

Also add a few more tests for prebuilts with no file specified.

Bug: 31800129
Test: m -j, make sure .soong.environment is not written to the source tree
Change-Id: I623bc114b2ff534c8df9fb3ce273e804711f8f05

android/config.go

@@ -152,8 +152,10 @@ func saveToConfigFile(config jsonConfigurable, filename string) error {
 }
 
 // TestConfig returns a Config object suitable for using for tests
-func TestConfig() Config {
-	return Config{&config{}}
+func TestConfig(buildDir string) Config {
+	return Config{&config{
+		buildDir: buildDir,
+	}}
 }
 
 // New creates a new Config object.  The srcDir argument specifies the path to