blast: fix registering callbacks

This is a better fix for b/134194071 than ag/7998544. The previous
patch only protected the call to notify the binder thread to send a
callback.

This patch has both a start registration and end registration call.
During that time, the transaction callback cannot be sent. This is
closer to a long term fix for the bug.

Bug: 134194071
Test: Switch between front and back cameras to make sure the app
    doesn't crash.

Change-Id: I2d20c13cc1c8d13e5a1340dfaa8cbbaa4d3a30ab