Check if string resources have set a "formatted" attribute

[BugsBeGone]

Example: <string name="test" formatted="false">100% Success!</string>
This will produce the following in template.pot:

#, c-format
msgctxt "test"
msgid "100% Success!"
msgstr ""

The main purpose of this PR is essentially to negate an erroneously-detected format string if the boolean is set to "false". Also in the unlikely case that a formatted string is not detected, you can force it to use c-format to with the boolean set to "true".

Unfortunately I can't find much in the way of official documentation for anything that actually uses formatted="true|false", but Android Studio certainly knows enough to autocomplete it as a boolean.

Something to note is that this attribute will not be carried over into specific language files (either .po or .xml). Personally I treat the .../res/values-<lc>/strings.xml files as auto-generated, and keep all the clever metadata in the .po files. I can't really see a benefit in explicitly setting the no-c-format gettext flag, but perhaps that will be useful for someone.

[BugsBeGone]

It would be nice to also make the auto-detection a bit more robust, but that seems like a bit of a rabbit hole. The SDK platform tools have a linter that tries to do this, and even that is going to catch a lot of false positives (note the comment starting line 816): https://android.googlesource.com/platform/tools/base/+/master/lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/StringFormatDetector.java#799

[miracle2k]

Makes sense, thanks.