fix: downgrade Android DFU library to 2.6.0 for V1 (#3)

This is the newest version that's working with micro:bit V1 (V2 is OK with 2.9 and 2.6) and matches what's used in the current micro:bit app.
This broke in 2.7 and is still broken in 2.10.1.
Just downgrading as a workaround for now.
There are some interesting changes to review including some that might be able to be reverted by setting options.

Author: microbit-grace

android/build.gradle

@@ -65,5 +65,5 @@ dependencies {
     androidTestImplementation "androidx.test.espresso:espresso-core:$androidxEspressoCoreVersion"
     implementation "androidx.core:core-ktx:$coreKtx"
     implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
-    implementation "no.nordicsemi.android:dfu:2.9.0"
+    implementation "no.nordicsemi.android:dfu:2.6.0"
 }