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/java/sdk_library.go b/java/sdk_library.go
index a5db56c..1152ac3 100644
--- a/java/sdk_library.go
+++ b/java/sdk_library.go
@@ -2070,6 +2070,11 @@
 }
 
 // from android.PrebuiltEtcModule
+func (module *sdkLibraryXml) BaseDir() string {
+	return "etc"
+}
+
+// from android.PrebuiltEtcModule
 func (module *sdkLibraryXml) SubDir() string {
 	return "permissions"
 }