enable R8 minify for release builds

[ovitrif]

Turn on R8 for Bitkit Android release builds and consume the UniFFI AARs that ship consumer keep rules.

Deliverable

• Set isMinifyEnabled = true on the release build type.
• Set isShrinkResources = true on the release build type.
• Keep JNI native method names and their descriptor classes in app/proguard-rules.pro.
• Set the Kotlin compiler flag -Xannotation-default-target=param-property.
• Consume released bitkit-core, ldk-node, vss-client, and paykit Android artifacts that include those consumer keep rules.

Acceptance

• A minified release build installs and exercises Lightning, on-chain, Paykit, VSS, and hardware-wallet flows.