Check block0 for remounting partition R/W

Add a function check_first_block to read block0 and output a message
on screen if the device has been remounted. The function is called
for version >= 4 only; it executes after a failing block verification
and before recovery attempts.

Bug: 21124327
Change-Id: I49dc0b861c702698896a2495ca094215705d4650
(cherry picked from commit 9dac7970)