apex: support prebuilt_firmware for vendor apexes

prebuilt_firmware module is one of many prebuilt_etc-like modules. When
it is soc-specific, it is installed in /vendor/firmware. Similarly, when
prebuilt_firmware is embeded in a vendor apex, installing it in
<apex>/firmware instead of <apex>/etc.

Bug: 162701747
Test: lunch sunfish-userdebug
      m && device boots && vibrator works
Change-Id: I00d28cde42259aaf8221e3897df77efc42b0c1ca
diff --git a/apex/apex.go b/apex/apex.go
index f3699b0..1c64328 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -1740,7 +1740,7 @@
 }
 
 func apexFileForPrebuiltEtc(ctx android.BaseModuleContext, prebuilt prebuilt_etc.PrebuiltEtcModule, depName string) apexFile {
-	dirInApex := filepath.Join("etc", prebuilt.SubDir())
+	dirInApex := filepath.Join(prebuilt.BaseDir(), prebuilt.SubDir())
 	fileToCopy := prebuilt.OutputFile()
 	return newApexFile(ctx, fileToCopy, depName, dirInApex, etc, prebuilt)
 }