Add reporting and command args to deflake tests

Allow tests extending DeflakeHostTestBase to be used as command-line
tools to find flakes:
 - Add support for deflake_run_count, deflake_single_run_timeout,
   and deflake_test command-line args to customize runs of the test
   package.
 - Add method-level reporting of the number of flakes for each test and
   the last stacktrace.

Passing arguments to atest is still inconvenient but works; see example
in the class javadoc.

Test: made tests flaky, run with atest, see failure report
Bug: 134730673
Change-Id: I23f8c2db939ab49e0a2f314573de6460cd110318