Create new legacy version for API 16-20 users

[Lineflyer]

Current release is only working for API>=21 according to smoke tests.
Therefore the minSDK has been raised to 21.

If we want to publish a version for API 16-20 users we need to update the legacy branch accordingly.
I am setting the foundation for this next days by:

• Conservering current legacy state into legacy_API15
• Updating legacy to the most current working state (seemingly before new settings have been introcuded)
• Cherry-Picking some later changes onto legacy (i.e. New icon, User Agent fix, Hint for text input)

When this is done we can try to work on a legacy release as described in #266 (comment)

[bekuno]

I propose to use the release state 2020.08.21 as legacy base.
The new release will only delivered to devices with API21 onwards. Older devices will therefore further run with the old release.

[Lineflyer]

@bekuno
I meanwhile updated legacy basically to 2020.08.21 with some on-top cherry-picked fixes for:

• New icon
• User agent to enable online maps
• Hint for the text input field in cartridges

It compiles and runs for me in emulator API 16.
@bekuno Could you doublecheck the modified build.gradle on legacy? I removed the versioning logic but kept other recent changes (e.g. dependencies, NDK version, etc.) Hope this is OK.

[bekuno]

I created PR #343 with two change suggestions.
The other changes are fine, imho.

[Lineflyer]

Let me reopen this to track until release of a legacy version.

[Lineflyer]

AFAIR from c:geo the sequence for publishing a legacy (see also: cgeo/cgeo#8602 (comment)) would then be:

• Create a release from legacy with a version number < version number from release
• Publish both versions in parallel (or start with legacy and then release

Logic should be: versionCode_oldrelease < versionCode_newlegacy < versionCode_normalrelease

[Lineflyer]

On the weekend my plan would be to do the following:

• Build the legacy version from legacy with a versionCode in between 20220102 (the problematic beta) and 20220110 (the to be pubslihed beta), e.g. 20220105
• Publish it
• Publish the current beta 20220110 as production release

This should make the legacy version available to API 16-20 and the current beta available to API21+

[Lineflyer]

Legacy version 2022.01.14-legacy (Code: 20220105) is part of the ongoing release. See #251