Multi-view: respect decoder limits — instance limit per codec, overload detection, pause cells beyond capacity #83

Hardware decoders have hard limits. On the AG-216, /vendor/etc/media_codecs.xml declares H.264: 16 instances, H.265: 4 instances, both 489 600 macroblocks/s. With 8+1 cells this can be exceeded, and today the result is silent: a black cell, or every cell dropping frames.

Substreams help, but they are not always available. The app should report the problem and manage the cells itself:

1. Instance limit: read MediaCodecInfo.CodecCapabilities.getMaxSupportedInstances() (Android 6+) for the hardware decoder of each codec. A cell that would exceed it does not start a decoder and shows e.g. Decoder limit: 4 × H.265. It starts automatically when another cell frees an instance.
2. Overload detection, by measurement: the declared macroblock budget is far too conservative on some boxes (Device testing: UDP, plain RTP and RTSP multicast (#70, #72, #73) #74: about 2.7× worked), so it is not used to refuse streams. Instead, the measured decoded/received fps (see the stats issue) is used: if cells keep falling behind (below about 85 % for several seconds), the decoder is overloaded.
3. Dynamic cell count: on overload, the highest-numbered active cell is paused, showing Paused: decoder overloaded, until the load fits. Paused cells come back when the grid is reopened. The declared budget is logged for reference.

Related: #80 (status in the cell), #76.