Clean up of screen texts and some lint warnings

This is a bunch of commits, some to fix lint warnings, some to follow Android style guides.

Some highlights:
* Starting at API 14, put `Cancel` buttons on the left, like the rest of Android. Also don't use button frames there, again like the rest of Android.
* Get rid of "Yes"/"No" dialog buttons. Use "Cancel"/<verb for whatever the dialog is about>.
* Removed the title of some dialogs, per Android [style guide](https://developer.android.com/design/building-blocks/dialogs.html). Especially the "Attention". "You are offline" doesn't require anything more. Surely no attention.
* Use more concise screen texts, based on the Android [writing style guide](https://developer.android.com/design/style/writing.html), for example
 * Don't say please. "*Attempts at politeness can annoy the user*"
 * Don't say "successful". That is implied. writing it sounds like the program is expecting a virtual pat on the back. "You were successful? That's nice. Well done, AnkiDroid!"
 * Generally more concise texts
 * One texts that annoyed me personally to no end: When you were forced to do a full sync, AnkiDroid did ask "Do you really want to download the server\'s collection? Be aware that this will overwrite your local collection." The correct answer was "No, i <eep>y well do not! I want you to merge the changes! But i guess you don't give me a choice." Replaced that with a simple "Overwrite your local collection?"

Look at the commits for the rest.