Only translate prefixed strings to keys / axes.

In order to maintain compatibility, we need to only translate strings
prefixed with KEYCODE_ or AXIS_ into the appropriate key codes /
axes. Otherwise things like `adb shell input` will convert 4 into
KEYCODE_4 whereas before it would convert it into KEYCODE_BACK.

Bug: 14807833
Change-Id: I2cb41aa55dbb2e3fd62664c1b944877290d7e4be