Improve error reporting and command handling of `bu` command.

Redirects error messages to stderr in addition to Log.e.
This makes errors visible when running bu from adb shell,
which could previously fail silently. For example, an
unknown backup flag error is now printed to the console.

Additionally, moves the command validation to an earlier phase.
This ensures that running bu without a valid command
correctly prints the usage instructions, which was previously
prevented by userId check.

Test: `adb shell bu` shows usage.
Test: atest BackupEligibilityHostSideTest
Bug: 420527847
Flag: TEST_ONLY
Change-Id: Idb948ca2699a8feb0296d346923db8ea99e92727