BTA Application registration refactor

This patch replace BTA state machine state associated with registration
of new application, together with BTA_GATTC_API_REG_EVT, and
tBTA_GATTC_API_REG. Instead, it uses closure to post registration task,
and callback for registration event.

Bug: 30622771
Test: sl4a ConcurrentBleAdvertiserTest GattConnectTest
Change-Id: I62d68485170ef5472237d62b285353f2c9bc8250