Tiny fixup.

There was this remote chance that user could ask to geofence here, then turn the blocking off and on again causing trouble in the main activity code. This prevents such situation from occurring.

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# On branch better_geofencing
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# Changes to be committed:
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#	modified:   res/layout/activity_main.xml
#	modified:   res/values/strings.xml
#	modified:   res/xml/preferences.xml
#	modified:   src/org/mozilla/mozstumbler/GPSScanner.java
#	modified:   src/org/mozilla/mozstumbler/LocationBlockList.java
#	modified:   src/org/mozilla/mozstumbler/MainActivity.java
#	modified:   src/org/mozilla/mozstumbler/NetworkUtils.java
#	modified:   src/org/mozilla/mozstumbler/Reporter.java
#	modified:   src/org/mozilla/mozstumbler/Scanner.java
#	modified:   src/org/mozilla/mozstumbler/ScannerService.java
#	modified:   src/org/mozilla/mozstumbler/ScannerServiceInterface.aidl
#	modified:   src/org/mozilla/mozstumbler/preferences/PreferencesScreen.java
#	modified:   src/org/mozilla/mozstumbler/preferences/Prefs.java
#
# Untracked files:
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#	.idea/
#	MozStumbler.iml
#	debug.log
#	juhst4dm.dtb.txt
#	local.properties
#	tiro5klz.2qi.txt