Change Telecom Excecutor to cached thread pool

This may help with perf issues (ex: CTS ) as more
modules within Telecom use this executor which is
passed into to them by TelecomSystem

(see example CTS bug on AVD https://b.corp.google.com/issues/276283203#comment5)

Fixes: 276966319
Change-Id: Id1ac319f8d8911d87fb1da7506ed0bd0b090f3a1
Test: telecom cts