Fail check-boot-jars if boot jar contains 0 .class files

Protect against an invalid boot jar, e.g. one containing .dex files is
used instead of .class files.

Test: add a prebuilt for framework-tethering
      Run `m check-boot-jars` with the first fix from
	  https://r.android.com/1341756 and the build fails due to no
	  .class files because a dex jar is used.
	  Run `m check-boot-jars` with all fixeds from
	  https://r.android.com/1341756 and the build works.
Bug: 158304459
Bug: 159112414
Change-Id: I0e8ebd318312949bc58ba7a5c89f9e265b8bedf2

core/tasks/check_boot_jars/check_boot_jars.py

@@ -49,8 +49,10 @@ def CheckJar(allow_list_path, jar):
   if p.returncode != 0:
     return False
   items = stdout.split()
+  classes = 0
   for f in items:
     if f.endswith('.class'):
+      classes += 1
       package_name = os.path.dirname(f)
       package_name = package_name.replace('/', '.')
       if not package_name or not allow_list_re.match(package_name):
@@ -58,6 +60,9 @@ def CheckJar(allow_list_path, jar):
                               ' not in the allow list %s of packages allowed on the bootclasspath.'
                               % (jar, f, package_name, allow_list_path))
         return False
+  if classes == 0:
+    print >> sys.stderr, ('Error: %s does not contain any class files.' % jar)
+    return False
   return True