bugreport/bugreportz: improve help/errors.

Fix bugreportz to report the usage error rather than ignore extra
arguments. Also fix the synopsis in the usage text to include -p.

Update the bugreport warnings to have the correct way to use bugreportz,
since we now know it doesn't actually take a filename and instead
chooses its own.

Bug: http://b/156667896
Bug: http://b/29448020
Test: adb shell bugreport
Test: adb shell bugreport --help
Test: adb shell bugreportz
Test: adb shell bugreportz file.zip
Change-Id: I10875067cf49a1d9869a4cb7a4051c309eed6b17