fixes #3331

Notifications from strangers are disabled by default in order to cope
with spam. On the other hand, this complicates contacting other users
for the first time, which leads to a bad user experience.

* This is sadly needed

* Enable instead of activate

This further reduces the minimum API level to 16, which should encompass
most users stuck on older versions of Android (mainly BlackBerry OS and
Jolla users).

Several issues reported by code analysis were fixed, mainly around issues
with layouts.

Message styling purposly doesn’t require a whitespace after a closing
tag to make something like ~un~believable work. However it also breaks
_Programmierer_innen_ and other example where the tag is repeated as a
non tag in the word.
Therefor we change the rules that if a closing tag is followed by a higher
order closing tag (a closing tag followed by an end block or white space)
we ignore the first closing tag. But only if we don’t read another tag open.

If conversion feature is not available we will never write to PEP
therefor it is not advisable to process PEP events; otherwise
the changes we do might not land in PEP.

simply ignoring PEP is probably better than dynamically removing +notify

This reduces the minimum SDK version to 18 (Android 4.3), which notably is
the last supported version for the BlackBerry OS 10.3 Android compatibility
layer.

This backfills missing class methods for `java.nio.charset.StandardCharsets`
and `java.util.Objects` for compatibility with platforms which do not support
these (mainly Android SDK versions <= 18).

Changes to this library will be implemented as additional commits.

this commit is just to make policies equal and independent on various android
versions. support for http might be removed in the future across all versions.