[Thread] add Android.bp for Thread flags

This is not required for code in the mainline module since the flag
infra is not supported for mainline. But we will need to write code
in UserManager and it needs to be guarded by new Thread flags.

Note that Merged-In is added to skip merging this CL into the
udc-mainline-prod branch because trunk stable flagging is not supported
there

Bug: 307679182
Test: m
Merged-In: I88c993680976d2e10d672bfdbf45119f2db13868
Change-Id: Id53fa3e7c9592b04f37f4cbd7dcd0499859e59ed