Remove unused AltBeacon dependency

Native Kotlin scanner uses BluetoothLeScanner directly, making the
AltBeacon library dead code. Removes dependency and proguard rule.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -59,11 +59,6 @@ android {
     }
 }
 
-dependencies {
-    // AltBeacon library for native beacon scanning
-    implementation("org.altbeacon:android-beacon-library:2.20.6")
-}
-
 flutter {
     source = "../.."
 }

android/app/proguard-rules.pro

@@ -1,6 +1,5 @@
 # Keep beacon library classes
 -keep class com.flutterbeacon.** { *; }
--keep class org.altbeacon.** { *; }
 
 # Keep Bluetooth classes
 -keep class android.bluetooth.** { *; }