screen_ui: Initialize text buffer

Zero initialize the text buffer to make recovery not
render garbage when showing the menu or messages.

Change-Id: I0dd0d357757f6b0fd52ad3b3617d42bb1b835245
ref-id: https://github.com/CyanogenMod/android_bootable_recovery-cm/commit/865ed4b0548c1570aebb276364334f2844f8cee5
Revert "sr: re-add in_touch"

This reverts commit 3a4db425f100e8f9e820389e37f0890b7b47b401.
Revert "sr: Touch UI"

This reverts commit 24eb48ebccac1e467db9337724c41e4bc3f234ea.

Conflicts:
	screen_ui.cpp