Reland: Move MutateApexTransition shortcut later

Returning from MutateApexTransition too early is allowing invalid
apex_available values to be ignored for modules that have no apex
variations.  Move the exit later so that apex_available checks
happen and platform variants are hidden consistently.

This relands I324de8090a9f1a874b5b22f20b8d62bf22c7d79c along with
fixes to make more tests visible to the platform.

Bug: 394664568
Test: builds
Change-Id: I161dcefb8c7465eb2bf1bd924fb9c6e4ff5dea85
diff --git a/cc/testing.go b/cc/testing.go
index 14a6b7a..c9d362d 100644
--- a/cc/testing.go
+++ b/cc/testing.go
@@ -193,7 +193,7 @@
 			},
 			apex_available: [
 				"//apex_available:platform",
-				"myapex"
+				"//apex_available:anyapex",
 			],
 			llndk: {
 				symbol_file: "libm.map.txt",
@@ -253,7 +253,7 @@
 			},
 			apex_available: [
 				"//apex_available:platform",
-				"myapex"
+				"//apex_available:anyapex",
 			],
 			llndk: {
 				symbol_file: "libdl.map.txt",