[Invisalign2] Make BTA_DmXXX synchronous

BTA_DmXXX APIs in bta_dm_sec_api.cc are currently implemented
as asynchronous by posting a task to main thread. They are
called from BTIF which are already running on the main thread.
Here they are posted a second time.

This change make them synchronous and avoid posting to main
thread again.

Bug: 311196228
Bug: 301661850
Test: atest net_test_btif_stack
Change-Id: I193f283125c56ce263b3e2e0fd4ff2f6fb6f4dd6