Add non-interactive system server benchmarks

This allows benchmarking system server when not sitting at the device. To use
it, launch the NonInteractiveSystemServerBenchmarkActivity and then wait awhile.

The benchmark results will be available at
/sdcard/Android/data/com.android.startop.test/files/benchmark.csv

Change-Id: If7697657228f6e631846fdb79a5a5acfdaf7fe65

• startop/apps/test/Android.bp[diff]
• startop/apps/test/AndroidManifest.xml[diff]
• startop/apps/test/README.md[diff]
• startop/apps/test/src/NonInteractiveSystemServerBenchmarkActivity.java[Added - diff]
• startop/apps/test/src/SystemServerBenchmarks.java[diff]