Use TCM context if application context is unavailble.

In TextClassificationManager, we use the application context to reduce
the likelihood of leaking the underlying Activity. Unfortunately, some
places (e.g. auto, system_server) don't actually provide us an
application context, so we need to fall back to whatever context was
passed in to TCM in the first place.

Bug: 80188317
Test: none
Change-Id: Ib4b2e6ed543d3ed56cefd8f56717855158cdc0fe