Keep APIs and signatures for telephony-common

Ensure the public API surface and associated signatures are preserved
during R8 optimization. Also relax the the proguard compat mode setting,
as it's no longer necessary with the new API safeguards.

Bug: 421510447
Bug: 215530220
Test: atest CtsCurrentApiSignatureTestCases
Flag: build.RELEASE_R8_FULL_MODE_BY_DEFAULT
Change-Id: Iba7033f75b0de6dc23e7d900d11c05b68b7ce62f