Restrict validateApexClasspathFragments to active apex prebuilts

This is a followup to https://r.android.com/3073624 that moved
validation of bcp jars to the top-level prebuilt apex. It is possible
that there can be multiple prebuilt variants of a mainline module and
each prebuilt might have a skew in bcp jars. This CL limits the check to
only the selected apex prebuilt.

Test: go test ./apex
Bug: 328578801
Change-Id: If225a1af6004b6584b86ec442f99672f0d1f8314
diff --git a/apex/prebuilt.go b/apex/prebuilt.go
index b2afa39..9ad5159 100644
--- a/apex/prebuilt.go
+++ b/apex/prebuilt.go
@@ -848,7 +848,9 @@
 
 func (p *Prebuilt) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 	// Validate contents of classpath fragments
-	validateApexClasspathFragments(ctx)
+	if !p.IsHideFromMake() {
+		validateApexClasspathFragments(ctx)
+	}
 
 	p.apexKeysPath = writeApexKeys(ctx, p)
 	// TODO(jungjw): Check the key validity.
@@ -1074,7 +1076,9 @@
 
 func (a *ApexSet) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 	// Validate contents of classpath fragments
-	validateApexClasspathFragments(ctx)
+	if !a.IsHideFromMake() {
+		validateApexClasspathFragments(ctx)
+	}
 
 	a.apexKeysPath = writeApexKeys(ctx, a)
 	a.installFilename = a.InstallFilename()