Add some comments for VNDK / vendor snapshots

Also some files are refactored:

- snapshot_prebuilt.go is separated from vendor_snapshot.go. Now
vendor_snapshot.go contains snapshot generation codes, while
snapshot_prebuilt.go contains module definition codes.

- Some helper functions are moved from snapshot_utils.go to util.go.

- Some ambiguous names of types and functions are renamed.

We still can add more detailed comments about the snapshots. They are to
be uploaded in follow-up changes, to avoid making this change too big.

Bug: 173474311
Test: generate vndk and vendor snapshot
Change-Id: I18fa837ccdf44a042b7a78e5c3df25fd2de96d95
diff --git a/cc/cc.go b/cc/cc.go
index 986bf2b..7baaa66 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -1595,7 +1595,7 @@
 
 		// glob exported headers for snapshot, if BOARD_VNDK_VERSION is current.
 		if i, ok := c.linker.(snapshotLibraryInterface); ok && ctx.DeviceConfig().VndkVersion() == "current" {
-			if isSnapshotAware(ctx, c, apexInfo) {
+			if shouldCollectHeadersForSnapshot(ctx, c, apexInfo) {
 				i.collectHeadersForSnapshot(ctx)
 			}
 		}