Cleanup: small code issues #35

Small items found during review, none of them user-visible on their own:

• proguard-rules.pro keeps com.openipc.decoder.Decoder$Frame, which no longer exists (Frame is now a top-level class).
• RtspClient.start() creates Executors.newFixedThreadPool(5) but only submits 2 tasks.
• FramePool mixes small RTP buffers and NAL buffers of up to 1 MiB. A large request discards whatever small frame it polls, and small requests take the large buffers, so key frames always allocate and the pool can hold about 50 MiB.
• takeScreenshot() compresses a full-resolution PNG on the UI thread, which causes a visible stutter.
• RtspClient (line ~303): indentation is misleading (SuspiciousIndentation lint error).
• MediaCodecManager hard-codes 1280×720 and KEY_MAX_INPUT_SIZE = 1 MiB in the input format. 4K streams with large I-frames may need more.