Add ability to override subscriber capabilities.

In a future set of CLs, NPMS will offer to override a handful of
capabilities on a per-subId basis.  Respond to these overrides
in the telephony code.

Add basic locking around collections that were unprotected
against cross-thread interactions.

Test: bit FrameworksNetTests:android.net.,com.android.server.net.
Test: bit FrameworksTelephonyTests:com.android.internal.telephony.dataconnection.DataConnectionTest
Bug: 64133169
Change-Id: Ia66bc5f420c3678bd940bc02c2adaefc7b9ecf90
(cherry picked from commit 92c66f10cbdb5f50c462c7be24c120b6edb04584)