minSdkVersion as low as possible

To allow as many users as possible to participate, the minimum Android version should be as low as possible. Since BLE is introduced in Android 4.3 (API 18), this would be the ideal minSdkVersion.

With the help of the Android linter, I will summarize the sections that would be highlighted as incompatible depending on the API version one would pick as a minSdkVersion.

API 21

• The library androidx.security:security-crypto only supports API 23. Likely, it could be replaced with the library Android-Vault, which uses the Android keystore to store the key as well.

As well as these trivial issues:

• PowerManager.isIgnoringBatteryOptimizations(String): battery optimizations were only introduced in API 23.
• AlarmManager.setExactAndAllowWhileIdle(int, long, PendingIntent): setExact(int, long, PendingIntent) would provide the same behavior up to API 19.

API 19

• A lot of Bluetooth APIs were apparently rewritten; those rewrites are only included in API 21. As such, using the old (deprecated) methods would require a rewrite of BleClient and BleServer as an alternative for older devices. Some calls in GattConnectionTask would have to be changed.

API 18

Trivial issues:

• AlarmManager.setExact(int, long, PendingIntent): set(int, long, PendingIntent) would give the same behaviour.
• Objects.hash(Object...): The call in DayDate could return the timestampRepresentation value as a hash directly.

[Vanuan]

New information from Android Exposure API:

Bluetooth functionality, including all broadcast and scanning for BLE beacons and local database storage, happens within Google Play services on-device. Your app will need to have the BLUETOOTH permission in its manifest, but does not require ACCESS_FINE_LOCATION or BLUETOOTH_ADMIN. For your app to work on a device, the device must be running Android version 5.0 (API version 21) or higher.

So the minimum version should be API 21.

[fynngodau]

@corrado4eyes I believe there should be an implementation independent from GMS, see #10.