Don't synchronize two objects at same time.

Since we will synchronize mConnections and mApps when we need operating
them. On getByConnId and remove(int id) function, they will synchronize
one object then call another function to synchronize another object. On
race conditions, it will deadlock when called getByConnId and remove(int
id) at same time.
This patch doesn't synchronize two objects at same time.

Bug: 185730059
Test: atest BluetoothInstrumentationTests
Tag: #stability
Change-Id: Id29e4f49fe96b40b5b19faf295c3965d299bbeba