add android r8 consumer keep rules

[ovitrif]

Ship R8 consumer keep rules in the Android AAR for the UniFFI/JNA FFI surface.

Deliverable

• Package consumer-rules.pro into the AAR as proguard.txt.
• Keep com.sun.jna.Structure$FieldOrder.
• Keep annotated com.synonym.bitkitcore struct classes, fields, and constructors.
• Keep com.sun.jna.* JNI names used by libjnidispatch.
• Keep classes that extend com.sun.jna.*, including com.sun.jna.ptr ByReference types.
• Keep JNA Structure fields and constructors in com.synonym.bitkitcore.
• Keep JNA Callback methods in com.synonym.bitkitcore.
• Keep UniffiLib and IntegrityCheckingUniffiLib native method names in com.synonym.bitkitcore.
• Fail bundleReleaseAar when those consumer keep rules are missing or empty.

Acceptance

• The release AAR contains a non-empty proguard.txt with those keep rules.
• A minified Bitkit build loads com.synonym.bitkitcore without an app keep for that package.

[ovitrif]

Reopening so the remaining consumer keep rules in #145 can close this issue.