RequestManageCredentials dark theme

Manual testing steps
* Enable dark theme via Settings > display
* Install TestDPC and select 'Request to manage credentials'
* Verify dark theme is displayed (including light text)

Bug: 177901172
Test: atest com.android.settings.security.RequestManageCredentialsTest
      manual testing
Change-Id: I6db1361ffcbf1e193673eb5ca7ef3f403d5023b1