LatinIME: Make some stuff themable

Delete, shift, tab, settings icons were not getting
redirected because the drawable was never requested and
theme chooser seems not to redirect initial icons.
(Not sure but this may be why the lockscreen isn't themable)

Also, I added functionality for keys with dark backgrounds to have
a different text color.

Finally, keyTextStyle is now an integer format so that
the integer variables can be redirected easily.

Change-Id: I5d391b30d0e01f8b3c367b792d2819c27d78a017