Tried making the use of periods more consistent.

Resolves https://www.transifex.com/otf/bitmask-android/translate/#hr/strings/152743232
" Yeah, the use of a period is pretty inconsistent. It does seem like in
most cases in which the imperative is used, no punctuation is used
however. "Do not show again", "Open navigation drawer", "Please enter
your password", "Remind me later" etc."