Add support of <APEX>_backing.txt in mixed build.

Bug: 239081456
Test: CIs
Change-Id: I8a8a5e606d5b1bc19294fc92cd469e7aa01cdb43
diff --git a/apex/apex.go b/apex/apex.go
index ff0f504..3676607 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -1908,8 +1908,7 @@
 		// TODO(b/239081457): Generate the bazel bundle module file from the apex rule.
 		a.bundleModuleFile = android.PathForBazelOut(ctx, a.Name()+apexType.suffix()+"-base.zip")
 		a.nativeApisUsedByModuleFile = android.ModuleOutPath(android.PathForBazelOut(ctx, outputs.SymbolsUsedByApex))
-		// TODO(b/239081456): Generate the backing.txt file from Bazel.
-		a.nativeApisBackedByModuleFile = android.ModuleOutPath(android.PathForBazelOut(ctx, a.Name()+"_backing.txt"))
+		a.nativeApisBackedByModuleFile = android.ModuleOutPath(android.PathForBazelOut(ctx, outputs.BackingLibs))
 		// TODO(b/239084755): Generate the java api using.xml file from Bazel.
 		a.javaApisUsedByModuleFile = android.ModuleOutPath(android.PathForBazelOut(ctx, a.Name()+"_using.xml"))
 		installSuffix := imageApexSuffix