Capture libeigen's header correctly

libeigen exports entire directory, but only two directories contain
actual exported headers. And some of headers have no extension. This
adds a special behaviour for external/eigen when capturing snapshot.

Exempt-From-Owner-Approval: trivial comment change

Bug: 157106227
Test: m vendor-snapshot captures libeigen's headers
Change-Id: I47830c8818a7fe804c8a769069804275b09e99e8
diff --git a/Android.bp b/Android.bp
index 7ccee6c..d7717e4 100644
--- a/Android.bp
+++ b/Android.bp
@@ -14,7 +14,12 @@
 
 cc_library_headers {
     name: "libeigen",
+
+    // Although entire directory is exported, only Eigen and unsupported/Eigen
+    // contain actual exported headers. When capturing header files for
+    // snapshot, only these two directories will be globbed.
     export_include_dirs: ["."],
+
     vendor_available: true,
     host_supported: true,
     apex_available: [