Add .gitignore and switch to AndroidX

[ufoq]

Most of users use this lib together with ads libs that already use AndroidX and it may be troublesome to keep old appcompat working with AndroidX, that's why it's better to just switch to it.

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[ufoq]

@googlebot I signed it!

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CLAs look good, thanks!

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[ufoq]

BTW it's weird to have dependency on appcompat just for annotations, maybe switching 'implementation' to 'testImplementation' is enough (this way there will be no dependency problems no matter what lib dev will use). I can make PR if you want.