Make 'adb backup' -f handling more bulletproof and more flexible

* "adb backup" as the entire command line no longer crashes

* The "-f filename" option can now appear anywhere in the command line.
  A trailing "-f" at EOL prompts an error message and usage summary.

Change-Id: I040ed73c2ca3687e265e35600eb3ab2b3c879695
1 file changed