Clean up run_unit_tests wordiness

Preserve word boundaries of command-line arguments, e.g. when the
script is called with "foo bar" an argument (including quotes) don't
break it up into "foo" "bar" before passing to adb shell, etc.

Fixed a few other style issues while in there, and removed the sole
bashism so the script can just use an ordinary shell.

Change-Id: I7f9b2ea08cd7fa35b692d8f7c49006bde6e562eb