Enable shrinking for ImsMediaService

Include the necessary keep annotations to preserve JNI code, and enable
basic shrinking of ImsMediaService. No service-specific classes
should be removed with this change.

Bug: 343759652
Test: presubmit + apkdiffstats
Flag: EXEMPT bugfix
Change-Id: I7a7f4b1394263deb4a7cf44642b829f0c75e9978
2 files changed