Fix command format of wm density

The getNextOption consumes an argument, which causes the format
becomes "wm density -d -d 2".

So change to not read the next argument too early. It only need
to get if a "-d" option is found.

Bug: 186124236
Flag: EXEMPT bugfix
Test: Create a virtual display.
      adb shell wm density -d 2
      It should output the density of the display
Change-Id: I41f7c4301693e189d2f819643dc9bea2e7d15439