DevicePolicyManager: ignore invalid proxy settings

Ignore exceptions thrown when invalid proxy settings are being
applied, in order to avoid breaking callers performing potentially
security-sensitive operations (like removing an ActiveAdmin)

Bug: 365975561
Test: manual
Flag: EXEMPT bugfix
Merged-In: I496b9ce7a0bbb1e6c245682be57b7357bb432d42
Change-Id: I496b9ce7a0bbb1e6c245682be57b7357bb432d42
(cherry picked from commit 2ea32931)