Fix bu command's showUsage() help text

The bu command's help message (showUsage()) was inaccurate.
It incorrectly stated that backup and restore operations used
file arguments (-f FILE and backup.adb) instead of standard
input/output.

This CL corrects the help text to reflect the actual behavior:

- backup writes to standard output, and restore reads from standard input.
- add missing flags (-widgets, -compress)

Bug: 420527847
Test: atest BackupEligibilityHostSideTest
Flag: TEST_ONLY
Change-Id: I0289301275f4632dfcc439294b69ecdb1dca9e87