Fix missing native library on Android

[simolus3]

To distribute libpowersync.so on Android, we used to depend on a :persistence project which in turn used to depend on com.powersync:powersync-sqlite-core, the core extension release for Android.

With the refactoring to androidx.sqlite, the :persistence project (and thus the dependency on the core extension) is gone. This breaks version 1.6.0 on Android, causing "libpowersync.so not found" errors.

To fix this, this:

1. Restores the dependency! I've tested the fix with both demos on an Android emulator.
2. While running core-tests-android, I stumbled across an issue with the proguard configuration of the bundled sqlite module. To avoid this impacting our users, we can add the missing proguard rules to the core package instead.
3. This removes unused jniLibs for the Android configuration - these are empty since the 1.6.0 release.

These fixes restore Android support for debug (tested with demos) and release (tested with core-tests-android). As a follow-up, I'll also look at running Android integration tests in our CI next week.