twrpapp: restore system mount to previous state

The current function to check for the existence of the TWRP app
in system unconditionally unmounts system once the check is
complete. Since the TWRP app check now occurs more frequently since
change 70e0a397e00c605cf7b01a5f5ce027dea84f1a14, system is now
getting unmounted every time the Advanced menu is entered, which
is undesirable if it was mounted in order to perform tasks from that

This patch first checks the state of the system mount point, and
once the check is complete, either unmounts system or leaves it
mounted, based on the initial state.

Change-Id: I7ad1f3e4e6cf3579134f1a9c5468ad90a142feae
1 file changed