libmodprobe: add thread safety annotations

Add thread safety annotations to let compilers check the thread safety
of `ModProbe` class. We used to have a data race issue in `ModProbe`
class. Utilizing thread safety annotation would mitigate future same
issues.

Bug: 400592897
Test: cuttlefish boots
Change-Id: I8c15394de5994b9f40ae8e2c53c6c8f8d7d36b31