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
Merged-In: I0e8ebd318312949bc58ba7a5c89f9e265b8bedf2
Change-Id: I0e8ebd318312949bc58ba7a5c89f9e265b8bedf2
(cherry picked from 92d41de8)