Ignore "@instance..." part of username when computing status length

[nikclayton]

In a status with a mention ("@[email protected]") only the "@foo" part should be included in the calculated status length. It wasn't, so the app was preventing people from posting statuses that should have been allowed.

Fix this.

• Lift the length calculation code in to a separate static function (easier and faster to test)
• Add a MentionSpan type, to reuse existing code for detecting mentions
• Fix a bug in FakeSpannable.getSpans() (it was returning the outer type, not the wrapped inner span)
• Add additional fast tests

The tests made sense under the components.compose.ComposeActivity package, so I also created that and moved the existing ComposeActivity tests there.

Fixes #3339

[nikclayton]

The necessity of the change to NoUnderlineURLSpan here is weird.

The symptom was that, under test, the .url property of the span was returning null, when debugging showed that it was definitely set.

I tried reproducing this behaviour in https://play.kotlinlang.org/#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 and couldn't.

The only difference I could think of is that URLSpan is implemented in Java, and maybe some critical type information is being lost in the toTypedArray() call in FakeSpannable.getSpans(), resulting in the null?

The problem also occurred running the tests under Robolectric.

The problem doesn't occur when the code's running in the app, only under test.