Add exported_bootclasspath_fragments to prebuilt_apex/apex_set

This is needed to allow a prebuilt_bootclasspath_fragment to be used
interchangeably with a bootclasspath_fragment in the
platform_bootclasspath module.

The platform_bootclasspath module depends on APEX specific variants of
bootclasspath_fragment modules. That works because the
bootclasspath_fragment modules are part of an apex and so have an APEX
specific variant which the platform_bootclasspath can specify.

Using a prebuilt_bootclasspath_fragment in place of a
bootclasspath_fragment requires that the prebuilt also has an APEX
specific variant.

Specifying exported_bootclasspath_fragments on a prebuilt_apex/apex_set
will cause it to create an APEX variant for the named module whcih will
allow it to be selected by the platform_bootclasspath module.

Bug: 186034565
Bug: 177892522
Test: m nothing
Change-Id: I7ddacc6498ec3a4a9f26c5f78b7f9a033e494d78
diff --git a/apex/deapexer.go b/apex/deapexer.go
index 1db13f9..9bc5720 100644
--- a/apex/deapexer.go
+++ b/apex/deapexer.go
@@ -44,9 +44,13 @@
 // module.`
 type DeapexerProperties struct {
 	// List of java libraries that are embedded inside this prebuilt APEX bundle and for which this
-	// APEX bundle will provide dex implementation jars for use by dexpreopt and boot jars package
-	// check.
+	// APEX bundle will create an APEX variant and provide dex implementation jars for use by
+	// dexpreopt and boot jars package check.
 	Exported_java_libs []string
+
+	// List of bootclasspath fragments inside this prebuiltd APEX bundle and for which this APEX
+	// bundle will create an APEX variant.
+	Exported_bootclasspath_fragments []string
 }
 
 type SelectedApexProperties struct {