Add apex_set module.

apex_set takes an .apks file that contains a set of prebuilt apexes with
different configurations. It uses extract_apks to select and install the
best matching one for the current target.

Bug: 153456259
Test: apex_test.go
Test: com.android.media.apks
Change-Id: I1da8bbcf1611b7c580a0cb225856cbd7029cc0a7

apex/builder.go

@@ -61,6 +61,7 @@ func init() {
 	pctx.HostBinToolVariable("zipalign", "zipalign")
 	pctx.HostBinToolVariable("jsonmodify", "jsonmodify")
 	pctx.HostBinToolVariable("conv_apex_manifest", "conv_apex_manifest")
+	pctx.HostBinToolVariable("extract_apks", "extract_apks")
 }
 
 var (