match_locale() will return false for empty locale string in the PNG
file. Also add a manual test to validate if a PNG file is qualified to
use under recovery.

Bug: 34054052
Test: recovery_manual_test catches invalid PNG files successfully & Locale_test passed

Change-Id: Id7e2136e1d8abf20da15825aa7901effbced8b03

Move recovery-refresh/persist tests out because these tests need special
steps to run. Also switch the constants to std::string.

Test: recovery_manual_test passed on an A/B device
Change-Id: I60b3ec6f094044945c3aafc1fae540896a6ddea6

Bug: 26962907
Change-Id: I5f80636af1740badeff7d08193f08e23f4e4fee1

Change-Id: Ie77a5584e301467c6a5e164d2c62d6f036b2c0c0

At the moment, this is the only difference in the sprout recovery
image. That's silly. Let's just improve the error handling slightly
and always have this option present.

Also make the obscure "<3e>" less unclear.

Also use "power button" as the default text rather than "enter button",
because it's been years since anyone had one of those. (Longer term we
should let subclassers tell us the keycode and we translate it to the
correct string.)

Also move the two "Reboot" options together, put "Power off" at the
bottom (and use that terminology, like the real UI, rather than
"Power down"), and use capitals throughout.

Finally, add a README.md with some useful instructions.

Change-Id: I94fb19f73d79c54fed2dda30cefb884426641b5c