add proguard rules

[ebsangam]

Solves issue on Android where user needs to add proguard rules. This PR will add proguard rules to the plugin.

[IVLIVS-III]

Did I understand correctly, that after merging this PR users of the device_calendar plugin no longer need to add that line to their proguard file anymore?

What happens if a user uses other rules in their proguard file, will they be merged by proguard?

If so, please also change the Readme, that it no longer says users must add that line to their proguard files.

[ebsangam]

Did I understand correctly, that after merging this PR users of the device_calendar plugin no longer need to add that line to their proguard file anymore?

What happens if a user uses other rules in their proguard file, will they be merged by proguard?

If so, please also change the Readme, that it no longer says users must add that line to their proguard files.

1. App will no longer needs to add proguard rules.
2. They will be merged.
3. Updated.

[IVLIVS-III]

LGTM