Handle CameraX use cases separately

[mzorz]

Fixes #50

Instead of binding all use cases to lifecycle at once on Activity startup, this PR creates and binds each use case as per the user needs, and unbinds the current use case when the user switches to another use case.

While this might produce some artifacts and small waits, it prevents the crash from happening on devices where a matching resolution for image still capture and resolution for video capture can't be found, thus ending up not finding a match for both cases at once (and crashing).

[mzorz]

Tested on emulators

• API 26 (Android 8.0) works alright ✅
• API 22 (Android 5.1.1) fixes the issue ✅ (some other issues appear later)
• API 24 (Android 7.0) -> crashes before, opening issue Crash on preview start on Android 7.0 (API 24) #79 ❌
• API 23 (Android 6.0 marshmallow) works alright ✅

[mzorz]

Merging this one, can be reviewed later when a PR for branch demo1/1.0.0.6-feedback is up

[aforcier]