app_process: Forward -cp/-classpath to ART.

* No new functionality was added, this was already possible via
  -Djava.class.path=classpath
* However, this makes it a bit more compatible with other tools
  that use -cp to invoke the main class.
* Also update the benchmarks README for correct up-to-date vogar
  instructions.

Test: vogar --mode app_process --benchmark frameworks/base/core/tests/benchmarks/src/android/os/ParcelBenchmark.java
Bug: 31807538
Change-Id: Idb04600fed3dd955437ccac832617dcfd1b52b63