Install appSet using InstallFileWithExtraFilesZip

This fixes a bug that only the primary output of an appSet is installed
when the appSet is part of a flattened APEX.

Bug: 247072627
Test: m GoogleExtServices on a device using flattened APEX.
system/apex/com.android.extservices.gms/priv-app/GoogleExtServices@TM
has these apks:

GoogleExtServices.apk  GoogleExtServices-arm64_v8a.apk
GoogleExtServices-hdpi.apk  GoogleExtServices-ldpi.apk
GoogleExtServices-mdpi.apk  GoogleExtServices-tvdpi.apk
GoogleExtServices-xhdpi.apk  GoogleExtServices-xxhdpi.apk
GoogleExtServices-xxxhdpi.apk

(before the fix, there only was GoogleExtServices.apk)

Change-Id: Icbc4dd002f856a3f751badec781ad132c423ac9b
diff --git a/apex/builder.go b/apex/builder.go
index ad8075b..e4c1673 100644
--- a/apex/builder.go
+++ b/apex/builder.go
@@ -922,10 +922,16 @@
 				installedSymlinks = append(installedSymlinks,
 					ctx.InstallAbsoluteSymlink(installDir, fi.stem(), pathOnDevice))
 			} else {
-				target := ctx.InstallFile(installDir, fi.stem(), fi.builtFile)
-				for _, sym := range fi.symlinks {
-					installedSymlinks = append(installedSymlinks,
-						ctx.InstallSymlink(installDir, sym, target))
+				if fi.class == appSet {
+					as := fi.module.(*java.AndroidAppSet)
+					ctx.InstallFileWithExtraFilesZip(installDir, as.BaseModuleName()+".apk",
+						as.OutputFile(), as.PackedAdditionalOutputs())
+				} else {
+					target := ctx.InstallFile(installDir, fi.stem(), fi.builtFile)
+					for _, sym := range fi.symlinks {
+						installedSymlinks = append(installedSymlinks,
+							ctx.InstallSymlink(installDir, sym, target))
+					}
 				}
 			}
 		}