Relax restriction on suffix of apex_set filename to include .capex

Compressed APEX files generated by the build system have the .capex
extension. This CL allows us using those .capex files with their actual
suffix.

Bug: 197258691
Test: manual + unit test added
Change-Id: I79173ef942326b48b5e659f1873d80f12b212339

apex/apex.go

@@ -1152,9 +1152,10 @@ const (
 
 const (
 	// File extensions of an APEX for different packaging methods
-	imageApexSuffix = ".apex"
-	zipApexSuffix   = ".zipapex"
-	flattenedSuffix = ".flattened"
+	imageApexSuffix  = ".apex"
+	imageCapexSuffix = ".capex"
+	zipApexSuffix    = ".zipapex"
+	flattenedSuffix  = ".flattened"
 
 	// variant names each of which is for a packaging method
 	imageApexType     = "image"