Migrate to WorkerThread and BackgroundThread

Remove a bunch of low-utilization threads from the Phone
process, instead moving standard priority work onto a new
worker thread, and background work such as metrics onto
the existing BackgroundThread.

Bug: 390244513
Flag: com.android.internal.telephony.flags.thread_shred
Test: atest FrameworksTelephonyTests
Test: manually verified CUJs on Panther
Change-Id: I4f5176921ee130a2f480575d636b2aaa9f278ff4