CarrierServiceBindHelper: context mismatch in bindService vs unbindservice

Two issues are observed due to the context mismatch:
1. unbindservice throw IAE with msg "Service not registered"
2. Binding connection leaks in various scenarios

To fix the issue, the context is created in the constructor and
used ever since.

Bug: 197274279
Test: atest CarrierServiceBindHelperTest
Change-Id: I0b6ebf88c5de71a8602f22d324cea91141c3c97c