Remove compat below API level 7

Just a little cleanup: we don't need compat below API level 7 anymore. Also, this fixes two issues I introduced in earlier pull requests: https://github.com/ankidroid/Anki-Android/pull/253/files#diff-0f8a52343a8a892b466c1fe842a39b1eR9 and https://github.com/ankidroid/Anki-Android/pull/240/files#diff-ca1d2ee63a25e3b499fcfcd8b7b67ecaL11 (extending the wrong compat classes).

As for Nooks, I checked and the Nook Color runs on API level 8 and the Nook Tablet runs on API level 10, so there shouldn't be any problem there.