[Java] Make default sdpSemantics NOT_SPECIFIED.

The default value of sdpSemantics is about to change from PLAN_B to
UNIFIED_PLAN. In order not to cause subtle bugs by applications that
depend on the default value being PLAN_B, we are temporarily making the
default NOT_SPECIFIED. Constructing with NOT_SPECIFIED causes the C++
layer to crash (https://webrtc-review.googlesource.com/c/src/+/242968).
This is in accordance to the publically announced plans:
https://groups.google.com/u/1/g/discuss-webrtc/c/SdoVP02eUIk

While we're at it, we're upgrading almost all unit tests to use Unified
Plan. However there are still two tests using Plan B for which I added
TODO comments to be dealt with later; not having an Android setup makes
it impossible to debug these efficiently.

Bug: webrtc:11121
Change-Id: Ib086186bee947d18d31b413e3aeba0cb247b377d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/246000
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Xavier Lepaul‎ <xalep@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35700}

sdk/android/src/jni/pc/peer_connection.cc

@@ -118,6 +118,9 @@ SdpSemantics JavaToNativeSdpSemantics(JNIEnv* jni,
   if (enum_name == "UNIFIED_PLAN")
     return SdpSemantics::kUnifiedPlan;
 
+  if (enum_name == "NOT_SPECIFIED")
+    return SdpSemantics::kNotSpecified;
+
   RTC_DCHECK_NOTREACHED();
   return SdpSemantics::kPlanB_DEPRECATED;
 }