Currently, when you hit 'A' for track select it give you a select all track select cursor.... to change it, you have to hold the shift key. Instead, allow multiple presses of "a" in a row to cycle through the different track select options.
I must agree with Jim.
After effects has shortcuts like this, (I believe it is called 'tapdance') but I don't think it's ever used for tools. I could be wrong.
Far better would be to give the single track selector tool its own unique shortcut.
Jim Simon commented
I would find this VERY annoying. I go to hit A, not sure if I did so I hit it again, and the tool changes.
Modifiers are by far the better method of changing tool operation.