Use SessionDescriptionInterface::GetType instead of type()

This patch migrates all internal call sites from the string-based
SessionDescriptionInterface::type() method to the enum-based GetType().

The existing type() method is scheduled for deprecation. Marking
`type()` as deprecated will need to wait a little longer however since
downstream code needs to be updated first.

- C++ tests and helpers now call GetType() and use SdpTypeToString
  for logging.
- The Android JNI function NativeToJavaSessionDescription is modified
  to accept the SdpType enum instead of a std::string.
- An Objective-C helper, typeForSdpType, is added to convert the
  C++ SdpType enum to the native RTCSdpType.

Bug: none
Change-Id: Ie08adb524da1ccae6dc70a215dea4708bbea4d7b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/409261
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45649}