Encrypt LE link immediately on reconnection

LE link must be encrypted immediately on connection if device are
already bonded.

This is a backport of ag/29056565, but the code needs to go in a
different location because that patch relies on recent feature work.

Ignore-AOSP-First: security
Test: mmm packages/modules/Bluetooth
Bug: 288144143
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:0064c0b351fc0902ebbf7bff2e84ce888abb396e)
Merged-In: I7147c837ecab6c67943fc6fd78a9949f3381df62
Change-Id: I7147c837ecab6c67943fc6fd78a9949f3381df62