Cache myki top up locations XML

[longzheng]

• Speed up time to load top up map
• Cache XML locally on disk
• If available, use on-disk cache to load map initially
• Also load online map XML and re-save it (check if diff size, then calculate diff)
• Also update Cordova-Googlemaps

Unfortunately the PTV server doesn't seem to support ETAGs or any HTTP fields that has a expiry check (always new expiry).