bluetooth: Add control for reading/writing of descriptor values to internals page.

Adds ValueControl to descriptor list and modifies it to handle reading
    and writing descriptor and characteristic values.

BUG=651282
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2649473002
Cr-Commit-Position: refs/heads/master@{#446586}
6 files changed