Add a path for 'external' time suggestions

Add a path for 'external' time suggestions to get to the time detector.

Enables form factor specific time sources. Example, Android Auto OS
can use 'external' time to interface with the vehicle HAL.

Bug: 177079827
Test: atest services/tests/servicestests/src/com/android/server/timedetector/TimeDetectorStrategyImplTest.java
Test: atest android.app.timedetector
Change-Id: I4f05dc9de6a9327ef7cf9200e4986164221d2fe8