Enable explicit R8 compat mode for telephony-common

This target contains API components where signatures must be preserved.
For now, explicitly enable R8 compat mode (the current library default)
to avoid any changes in behavior when we switch the global default to
R8 full mode.

A follow-up change will update the Proguard config to preserve API
signatures and semantics even with R8 full mode.

Bug: 421510447
Test: atest CtsCurrentApiSignatureTestCases
Flag: EXEMPT preserving existing behavior
Change-Id: I73d21595c147e4e199b6f44a60a207ce33ef1f36