Fix incorrect notification about running out of storage space.

Previously, if there were problems generating the boot image, we would
display a notification saying the device is out of storage space,
regardless of whether the device was actually out of storage.

This change makes it so we only display the notification if the device
has less than 250MB when the boot image is missing.

Test: Verify the following sequence does not display the notification:
$ adb shell stop
$ adb shell rm /data/dalvik-cache/arm/*.oat
$ adb shell setprop dalvik.vm.extra-opts -Xnoimage-dex2oat
$ adb shell start

Test: Verify the following sequence does display the notification:
$ adb shell stop
$ adb shell rm /data/dalvik-cache/arm/*.oat
$ adb shell setprop dalvik.vm.extra-opts -Xnoimage-dex2oat
$ adb shell setprop debug.freemem 104857600
$ adb shell start

Bug: 28931369
Change-Id: Ia2291bf674af063a55f339d2a0411e670ca16ba2