Compile under L, use support annotations

Change just enough to compile under L, and replace the @Nullable and
@NotNull annotations with those from the android support library.
Materialize main activity

Switch to using AppCompat library and themes, so that everything looks
all material-y. Many activities left to convert, but now I know better
what I'm doing.
More progress toward L

Started on TaskActivity. Having trouble with getting the Toolbar to act
as an ActionBar.