Fix issues with bp transformation

Returning nil from transformPropertySet in order to remove it did
not work because it ends up comparing as (*bpPropertySet, nil) and
not an untyped nil which causes the test against nil to fail.

This change adds tests to check that returning nil will delete a
property/property set from the containing property set and fixes the
code so that it passes the tests. It extracts common code to transform
a property set and its contents as well as code for creating new
property sets.

Bug: 148933848
Test: m nothing
Change-Id: I35dc3c39c76e701821891622615c09b094cf697f

Android.bp

@@ -536,6 +536,7 @@ bootstrap_go_package {
         "sdk/update.go",
     ],
     testSrcs: [
+        "sdk/bp_test.go",
         "sdk/cc_sdk_test.go",
         "sdk/exports_test.go",
         "sdk/java_sdk_test.go",