Get the themes outside of the resource lock

According to the documentation surrounding the resource manager
lock, the lock should _not_ be held when calling back into the
activity manager. The process of getting the system context [in
order to obtain the theme] _may_ call back into the activity
manager.

Change-Id: Ibddf728a39e651f0b3ec1e2f56e09da233b963c3
Fixes: 71592993
Test: Manual