Move to a shared Worker Thread

Create a standard priority worker thread that can be
used to offload work that shouldn't block the main thread
but should be done at normal (non-background) priority.

Bug: 390244513
Flag: com.android.internal.telephony.flags.thread_shred
Test: atest FrameworksTelephonyTests
Test: manually verified CUJs on Panther
Change-Id: Id82e2a9e3c42b7706718a4178f6c57379eabb718