ClassZeroActivity: Holo it

Change-Id: I098866d61145863c561c3bdc3f25d38254906f1d
Fix inconsistent back button behavior

By default, a press of the back button will only take effect when the
button is successfully released (the user releases the button while
still focused on it), not when the button is still triggered and pressed
down. To avoid this inconsistent behavior, execute the custom back
logic in the onBackPressed callback instead of onKeyDown.

Change-Id: Id516bc523b625a7ca42ea3ff580892cdf28f770f
ClassZeroActivity: Queue messages instead of displaying them all at once

10 out of 9 doctors recommend not piling up infinite top activities

Change-Id: Id7a2faa6997036507acad38f43fe17bf1f6a42cd

ClassZeroActivity: Resolve some possible NPEs, and refactor common code

Match up to the final version merged upstream

Change-Id: I215188f5ea508b3c0b69189c1bf6ccc7601eed7b