Use prebuilt dx for PDK builds

The source for dx may not be available in PDK builds, use the
prebuilt one from prebuilts/build-tools instead.

Bug: 67663308
Test: m TARGET_BUILD_PDK=true
Change-Id: I9090b5190539f901fc05264a472133c12d4ea2a1

android/config.go

@@ -496,6 +496,10 @@ func (c *config) UnbundledBuild() bool {
 	return Bool(c.ProductVariables.Unbundled_build)
 }
 
+func (c *config) IsPdkBuild() bool {
+	return Bool(c.ProductVariables.Pdk)
+}
+
 func (c *config) DevicePrefer32BitExecutables() bool {
 	return Bool(c.ProductVariables.DevicePrefer32BitExecutables)
 }