Support androidx and fix iOS crash

[HarelM]

Fixes #21 - support androidx.
After merging this I suggest to release a version with an incremented major number i.e 4.x since this is a breaking change.

[HarelM]

I've also fixed the iOS crash according to #18 as part of this PR.

[dpogue]

Thanks for the pull request and for following up with me about it.

My plan is to do this in a few steps:

1. Merge only the iOS 15 fix to master and release v3.0.2
   This will fix the iOS issue while maintaining compatibility with existing apps.

2. Cherry pick the iOS 15 fix to v2.x and release v2.0.2
   As requested by some of the folks in Fix issue with invalid callback scheme #19.

3. Merge the AndroidX changes and release a new major version v4.0.0
   Moving to AndroidX is a breaking change, and requires bumping the minimum Cordova Android version to v9.0.0.

Can you check if the CustomTabs stuff in plugin.xml needs to change for AndroidX?

cordova-plugin-oauth/plugin.xml

Lines 85 to 87 in 71e4f0c

	<preference name="ANDROID_SUPPORT_CUSTOM_TABS_VERSION" default="28.+" />
	<framework src="com.android.support:customtabs:$ANDROID_SUPPORT_CUSTOM_TABS_VERSION" />

[HarelM]

I couldn't find any documentation on that unfortunately...

[dpogue]

These have been split up across the various tags/versions as described above and merged, so I'm going to mark this as closed. Thanks for pushing this along.