enable aapt for unbundled builds

It was available for unbundled builds when the build rules were in mk form and aapt2 is enabled for unbundled, so there's not a strong reason why aapt should be disabled for unbundled build

Change-Id: I6589eafbe8d2e423c9ac925b3614763fa6a511ac
Fixes: 217747798
Test: local compile

tools/aapt/Android.bp

@@ -61,16 +61,6 @@ cc_defaults {
             enabled: true,
         },
     },
-
-    // This tool is prebuilt if we're doing an app-only build.
-    product_variables: {
-        pdk: {
-            enabled: false,
-        },
-        unbundled_build: {
-            enabled: false,
-        },
-    },
 }
 
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