disable async reboot during package installation

The default recovery UI will reboot the device when the power key is
pressed 7 times in a row, regardless of what recovery is doing.
Disable this feature during package installation, to minimize the
chance of corrupting the device due to a mid-install reboot.  (Debug
packages can explicitly request that the feature be reenabled.)

Change-Id: I20f3ec240ecd344615d452005ff26d8dd7775acf