Explicitly define Kotlin api/language versions

[fractalwrench]

Goal

Explicitly defines that bugsnag-android uses Kotlin 1.3 to build, and should target API compatibility with 1.3. This is achieved by setting the compiler options.

Future updates may increase the api/language version used - for now this changeset just defines what is already in use.

Testing

Relied on existing test coverage. I also logged out the properties of kotlinOptions to confirm that the apiVersion and languageVersion were set on each task, whereas they were not before.

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1353.03	1167.83
arm64_v8a	483.72	299.4
armeabi	-492.59	-21.55
armeabi_v7a	455.06	270.74
x86	520.57	340.35
x86_64	508.29	323.97

Generated by 🚫 Danger

[lemnik]

LGTM