Only enforce caller is root for special commands

Commands like "adb shell cmd dreams", "adb shell cmd dreams dump",
and "adb shell cmd dreams help", should all work. Also, when the
other commands are called without being root, print the exception
to the PrintWriter instead of to logcat where it may not be
noticed.

Fix: 272000370
Change-Id: Ida3c9bdb8b54a8897fe8261222f4cb9b1a08e0e4
Test: adb shell cmd dreams help # accepted
Test: adb shell cmd dreams start-dreaming # rejected