apollo: wait for external storage

Avoid crash on boot in ImageCache calling getExternalStorageDir
Add some wait time and if all goes wrong default to the
internal storage dir

See also https://github.com/CyanogenMod/android_packages_apps_Apollo/commit/d06510a2cbb723f092fbdbdc8a1a7b7f39f00c34

Change-Id: I52f5b7646a2e4701b514f03e1a8d6ff43283c46d