Add Sentry.crashedLastRunEventId() to return event id of last crashed run

[romtsn]

Meta issue: https://github.com/getsentry/team-mobile/issues/76

We could modify crashedLastRun marker file and also store the last event id.

[jamescrosswell]

Just adding a use case (we'd like to implement this in the sentry-dotnet Android Sample):

We would like to show a dialog to collect feedback from users when we know the application crashed. For example "We noticed there was a problem last time you ran the application. Do you want to tell us what happened and maybe we can help you out?"

Ideally feedback related to crashes would be associated with the Sentry event (these events contain a lot of context that users won't be able to articulate). So it would be great to have the Event ID available when OnCrashedLastRun returns true.