Small improvements to code quality and readability

Add annotations to methods and parameters.
Replace Jetbrains' @NotNull with Androidx' @NonNull annotatiation.
Make class variables static if possible.
Use constants for some Strings.
Simplify if conditions.

app/src/main/java/us/shandian/giga/io/FileStreamSAF.java

@@ -108,10 +108,12 @@ public class FileStreamSAF extends SharpStream {
         return true;
     }
 
+    @Override
     public boolean canSetLength() {
         return true;
     }
 
+    @Override
     public boolean canSeek() {
         return true;
     }
@@ -131,10 +133,12 @@ public class FileStreamSAF extends SharpStream {
         out.write(buffer, offset, count);
     }
 
+    @Override
     public void setLength(long length) throws IOException {
         channel.truncate(length);
     }
 
+    @Override
     public void seek(long offset) throws IOException {
         channel.position(offset);
     }