[android] Warn users about slow SD cards that may make impression that OM is "slow"

We see complaints from many reviews that OM is "slow" on Android devices with SD cards. Unfortunately, storing maps on SD is almost always slower than storing maps in the internal device memory. Not only SD cards may be slow themselves, but also recent Android 11 and above changes have significantly slowed down reading SD card contents.

The idea is to detect when users are using an SD card (or even better - a slower SD card), and show an appropriate warning message at the right time (not an annoying one), something like this:

You are storing maps on an SD card. If Organic Maps works slower than you expect, try to store maps in the internal memory, or try to use a newer/faster SD card.

CC @oleg-pah