[Fix] Fix Sui not working with R8 full mode.

libsu has:

-keep,allowobfuscation class * extends
com.topjohnwu.superuser.ipc.RootService { *; }

However since Shizuku-API doesn't use a particular base class and only
requires the class to be extending IBinder, there's no good proguard
rule that can be bundled within that library. So just mark our class
with @keep for simplicity.

Fixes: #1034

app/src/main/java/me/zhanghai/android/files/provider/root/SuiFileServiceLauncher.kt

@@ -11,6 +11,7 @@ import android.content.pm.PackageManager
 import android.os.Build
 import android.os.IBinder
 import androidx.annotation.ChecksSdkIntAtLeast
+import androidx.annotation.Keep
 import androidx.annotation.RequiresApi
 import kotlinx.coroutines.TimeoutCancellationException
 import kotlinx.coroutines.runBlocking
@@ -145,6 +146,7 @@ object SuiFileServiceLauncher {
     }
 }
 
+@Keep
 @RequiresApi(Build.VERSION_CODES.M)
 class SuiFileServiceInterface : RemoteFileServiceInterface() {
     init {