Add checkstyle

[Stypox]

• I carefully read the contribution guidelines and agree to them.

This PR adds checkstyle to the project. The motivation for doing this, and doing it in a single PR is the following: there have been many PRs lately that should fix one thing but while they are at it they also fix the formatting, making it far more difficult to review. That's why from now on any misformatting will be reported as an error (only test tasks though, not compileJava tasks so that we don't overload Jitpack's servers with useless checkstyle checks).

• In commits "...downloader.Request", "...DashMpdParser", "...DonationLinkHelper", "...JsonUtils" I refactored some other small things (explained in the commits' descriptions). I feel bad for this as they are somewhat unrelated changes, but it would probably have taken me more time to reformat than to refactor ;-)
• In commits "...MediaFormat..." and "...NewPipe..." instead of reformatting some methods I converted them to Java 8 Streams API altogether for the same reason as above ;-)
• The checkstyle.xml I used is the same as NewPipe minus some Javadoc-related stuff. There is no need for strict rules over comments here in the extractor, as sometimes javadocs are just needed to clarify a small thing and having empty/meaningless @param or @throws is useless.
• I had to use checkstyle version 9.3 instead of 10.0 as the latest requires compile SDK 11
• While doing this PR I also found other places where the streams API could be used, but I refrained from adding unrelated changes here and will open a PR after this is merged. Please remind me if I forget when merging (and a reminder for myself: the local branch is called fix-warnings).

After this PR is merged I would propose to BAN any reformatting unrelated to the modified code in extractor PRs, unless the PR contains only reformats. The ways to format things that are not constrained by checkstyle are pretty subjective (e.g. whether to put multiple .foo().bar().baz() on the same line or split them), so it does not make sense to keep reformatting them, and in any case it should be done in separate PRs.

I hope this PR will be merged soon otherwise... many conflicts to solve ;-)

[litetex]

Good work! (；^＿^)ッ☆(　゜u゜)

Took me some time to review this... but mostly LGTM, only some minor things:

PS: Also please rebase the PR

[litetex]

Note: This class definetly needs a refactoring after we merged this.

[litetex]

Fixed above problems myself.

Note: This PR breaks backwards compatibility with NewPipe → new major/minor release; no patch release + PR at NewPipe required!

[litetex]

All Mix-Playlist tests are failing:

Could not get playlistData
org.schabi.newpipe.extractor.exceptions.ExtractionException: Could not get playlistData
	at org.schabi.newpipe.extractor.services.youtube.extractors.YoutubeMixPlaylistExtractor.onFetchPage(YoutubeMixPlaylistExtractor.java:100)
	at org.schabi.newpipe.extractor.Extractor.fetchPage(Extractor.java:60)
	at org.schabi.newpipe.extractor.services.youtube.YoutubeMixPlaylistExtractorTest$ChannelMix.setUp(YoutubeMixPlaylistExtractorTest.java:356)
	...

Also happens on the dev-Branch → can be ignored

[TobiGr]

Do you want to merge this before #810? I would not suggest it.

[litetex]

@TobiGr
Yes

[litetex]

Current test-results (mock):

Reasons:

• HlsOnlyStreams → Peertube instance (https://tilvids.com) is down (Status code: 502)
• see Fixed tests (+ Youtube shorts in channels) #813 for the rest