Add keycombination and double tap key commands

This will provide a convenient way to test combination key and single
key gesture without connecting a physical keyboard.

New commands:
'keycombination' : press multiple keys at same time.
'keyevent --doubletap' : double click with the same key.

Also fix the command 'keyevent --longpress' to let it waiting until long
press timeout instead passing the FLAG_LONG_PRESS flag, it's due to some
behaviors may check the event time.

Test: adb shell input keycombination keycode1 keycode2
Test: adb shell input keyevent --doubletap keycode
Test: adb shell input keyevent --longpress keycode
Bug: 162194917
Bug: 162194389

Change-Id: Ibe4d941ebe53cc5738db3ba9b59dffeeb31a7308