fix: tighten span tag parsing

[IgorEisberg]

Fixes #4059
Fixes: #2890

This makes the splitting of the attributes part of a StyledString span's tag stricter.
Unfortunately, the way these tags are encoded is very ambiguous, so we make a best guess approach to cover all known cases.

Example: <span href="https://test.com/1234#param=val" style="font-family: serif; font-size:12px;">
Is encoded as: span;href=https://test.com/1234#param=val;style=font-family: serif; font-size:12px;

The keys have a specific set of valid characters, but the values can contain any character, including the separators ; and =.
We're splitting on ; but with a lookahead for a valid key followed by =.
This is not 100% fail-proof and won't decode accurately if a value contains a ; followed by a valid key format, but at least it won't crash (discussed in the comment below).

Added test cases as well.

[IgorEisberg]

This can't handle the case in #2890 (doesn't crash now though).

a;href=intent:#Intent;action=android.settings.SYSTEM_UPDATE_SETTINGS;end

Here, ";action=" is wrongly identified as another attribute key, but it's part of intent:.

However, that tag is not valid in ARSC to begin with and is a translation mistake.
Android itself only reads the "intent:#Intent" part and the rest is assumed to be a separate subtag.
https://android.googlesource.com/platform/frameworks/base/+/refs/tags/android-16.0.0_r4/core/java/android/content/res/StringBlock.java#507
Printing the spans in the spanned string on Android outputs URLSpan{URL='intent:#Intent'}, and action is ignored because it's unknown.
As you can see, these span tags are not HTML, just predefined HTML-like tags translated explicitly:
https://android.googlesource.com/platform/frameworks/base/+/refs/tags/android-16.0.0_r4/core/java/android/content/res/StringBlock.java#356

The default string escapes the tag (< instead of <) and then programmatically parsed using Html.fromHtml(), which is the correct way to deal with this special case. This PR can be merged as-is for now.

Improperly translated string, for example:

<string name="assistant_dictation_ineligible_android_old_version">Pokud chcete použít hlasové zadávání pomocí Asistenta, aktualizujte systém Android. <a href="intent:#Intent;action=android.settings.SYSTEM_UPDATE_SETTINGS;end"> Aktualizovat </a> <br/><br/><a href="https://support.google.com/gboard/answer/11197787#update_app"> Další informace </a></string>

The proper default string, for reference:

<string name="assistant_dictation_ineligible_android_old_version">To use Assistant voice typing, update your Android System. &lt;a href=intent:#Intent;action=android.settings.SYSTEM_UPDATE_SETTINGS;end> Update &lt;/a> &lt;br/>&lt;br/>&lt;a href=https://support.google.com/gboard/answer/11197787#update_app> Learn more &lt;/a></string>