Skip instrumenting non-class files

Jacoco normally recurses into .jar files, but we don't want it to.
Some .jar files may be intentionally corrupt test files.

Test: m -j showcommands dist EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true SKIP_BOOT_JARS_CHECK=true WITH_DEXPREOPT=false cts google-tradefed-all general-tests tests
Bug: 68056327
Change-Id: I1550e3acafd7cd9a65164742370e051d3eb3bf69