Auto-Instrument onCreate and onStart in Activity transactions

[romtsn]

Description

We could create spans for each of the callbacks to better surface the performance of each method if they involve some heavy computations (similar to how sentry-cocoa does it for viewDidLoad, viewDidAppear and so on).

Android API 29 and above have new callbacks in the AcitivtyLifecycleCallbacks class (onActivityPreCreated/onActivityPostCreated and friends), so we could utilize them when implementing it.

[markushi]

Let's start small and use the existing API 29+ callbacks in the first iteration. And it could be a good chance to refactor our ActivityLifecycleIntegration.

[stefanosiano]

Let's schedule it for the next quarter @kahest