apex: prebuilt_firmware installs in /etc/firmware

Installation path of prebuilt_firmware varies accoding to target
partitions. When it is for system, it installs a file in
/system/etc/firmware while it installs in /vendor/firmware for vendor.

We'd better be consistent about installation path when it is
for APEXes regardless of target partition. Otherwise, ueventd would need
to scan both /apex/*/etc/firmware and /apex/*/firmware.

Having /etc prefix for prebuilt modules helps module owners to predict
the layout of the contents.

Bug: 162701747
Bug: 167942098
Test: soong tests
Test: loading vibrator firmware from vibrator apex (sunfish)
Change-Id: I7a7105026426f8a7a156bc947304930f761c81f3
diff --git a/etc/prebuilt_etc.go b/etc/prebuilt_etc.go
index 5dd2a86..8e35679 100644
--- a/etc/prebuilt_etc.go
+++ b/etc/prebuilt_etc.go
@@ -172,13 +172,7 @@
 }
 
 func (p *PrebuiltEtc) BaseDir() string {
-	// If soc install dir was specified and SOC specific is set, set the installDirPath to the specified
-	// socInstallDirBase.
-	installBaseDir := p.installDirBase
-	if p.SocSpecific() && p.socInstallDirBase != "" {
-		installBaseDir = p.socInstallDirBase
-	}
-	return installBaseDir
+	return p.installDirBase
 }
 
 func (p *PrebuiltEtc) Installable() bool {
@@ -205,7 +199,13 @@
 		ctx.PropertyErrorf("sub_dir", "relative_install_path is set. Cannot set sub_dir")
 	}
 
-	p.installDirPath = android.PathForModuleInstall(ctx, p.BaseDir(), p.SubDir())
+	// If soc install dir was specified and SOC specific is set, set the installDirPath to the specified
+	// socInstallDirBase.
+	installBaseDir := p.installDirBase
+	if p.SocSpecific() && p.socInstallDirBase != "" {
+		installBaseDir = p.socInstallDirBase
+	}
+	p.installDirPath = android.PathForModuleInstall(ctx, installBaseDir, p.SubDir())
 
 	// This ensures that outputFilePath has the correct name for others to
 	// use, as the source file may have a different name.