Upgrade target sdk to 30 and associated changes

[mrsarm]

(Like medic/cht-android#164)

Update targetSdkVersion to 30 along with a range of other requirements, documented here.

New apps will require this by August 2021
App updates will require this by November 2021

[garethbowen]

@mrsarm We don't distribute cht-gateway via the Play Store so I don't think those restrictions apply here. The reason we don't is because it relies on SMS permissions which will get us delisted anyway.

These improvements are still good, but not a high priority IMO.