e2ee: sync key ring size and packet format

this uses the full four bits in the wire format. The wire format
looses the (currently not implemented) extensibility with variable
length keys.

doc/e2ee.md

    +^+-------------------------------------------------------+ +
    | |                 Authentication Tag                    | |
    | +---------------------------------------+-+-+-+-+-+-+-+-+ |
-   | |    CTR... (length=LEN + 1)            |S|LEN  |0| KID | |
+   | |    CTR... (length=LEN + 1)            |S|LEN  |KID    | |
    | +---------------------------------------+-+-+-+-+-+-+-+-+^|
    |                                                           |
    +----+Encrypted Portion            Authenticated Portion+---+

modules/e2ee/Context.js

 import { isArrayEqual } from './utils';
 
 // We use a ringbuffer of keys so we can change them and still decode packets that were
-// encrypted with an old key.
-const keyRingSize = 3;
+// encrypted with an old key. We use a size of 16 which corresponds to the four bits
+// in the frame trailer.
+const keyRingSize = 16;
 
 // We copy the first bytes of the VP8 payload unencrypted.
 // For keyframes this is 10 bytes, for non-keyframes (delta) 3. See
             // but we put it at the end.
             //                                             0 1 2 3 4 5 6 7
             // ---------+---------------------------------+-+-+-+-+-+-+-+-+
-            // payload  |    CTR... (length=LEN)          |S|LEN  |0| KID |
+            // payload  |    CTR... (length=LEN)          |S|LEN  |KID    |
             // ---------+---------------------------------+-+-+-+-+-+-+-+-+
             const counter = new Uint8Array(16);
             const counterView = new DataView(counter.buffer);
      */
     async decodeFunction(encodedFrame, controller) {
         const data = new Uint8Array(encodedFrame.data);
-        const keyIndex = data[encodedFrame.data.byteLength - 1] & 0x7;
+        const keyIndex = data[encodedFrame.data.byteLength - 1] & 0xf; // lower four bits.
 
         if (this._cryptoKeyRing[keyIndex]) {
             const counterLength = 1 + ((data[encodedFrame.data.byteLength - 1] >> 4) & 0x7);