Use EncryptedSharedPreferences for HMAC key storage

Replace plain-text SharedPreferences with AES256-GCM encrypted prefs
via androidx.security:security-crypto. Adds PrefsHelper utility to
centralize encrypted prefs access across MainActivity, SettingsActivity
and UsbKeyReceiver.

Reviewed in codereview.md (recommendation #2).

android/app/build.gradle

@@ -42,4 +42,5 @@ dependencies {
     implementation 'io.github.dkaukov:esp32-flash-lib:1.1.17'
     implementation 'com.eclipsesource.minimal-json:minimal-json:0.9.5'
     implementation 'org.slf4j:slf4j-nop:2.0.16'
+    implementation 'androidx.security:security-crypto:1.1.0-alpha06'
 }