androidbp: simplify translation by evaluating all expressions in Blueprint

Translation is getting complicated because the expressions supported
by Blueprint are difficult to support in Make.  Modify androidbp
to use context aware parsing so it can evaluate all expressions at
parse time, so it only needs to deal with constant values.

Change-Id: I57047645fb48475baecd0361f78a93ec0a26011e

common/defs.go

@@ -27,10 +27,12 @@ var (
 	cpPreserveSymlinks = pctx.VariableConfigMethod("cpPreserveSymlinks",
 		Config.CpPreserveSymlinksFlags)
 
+	srcDir = pctx.VariableConfigMethod("srcDir", Config.SrcDir)
+
 	androidbpCmd = filepath.Join(bootstrap.BinDir, "androidbp")
 	androidbp    = pctx.StaticRule("androidbp",
 		blueprint.RuleParams{
-			Command:     androidbpCmd + " $in $out",
+			Command:     androidbpCmd + " ${srcDir}/Android.bp $in $out",
 			Description: "androidbp $out",
 		})