Enrich event with Android context on the Dart layer

[marandaneto]

Description

Relates to getsentry/team-mobile#11

Closes #1205

[marandaneto]

PR should use the v8 branch as the base branch.
PR can follow getsentry/sentry-react-native#3170 as an example, @krystofwoldrich and @markushi can help here with reviews since they worked on the implementation for RN and Android.