fix: Grab content description before posting to a handler

It's possible for the handler to execute when the context is no longer
valid, so grab the description early before posting, using the fragment
context available when the pref is first initialized.

Fix: 407904737
Flag: EXEMPT minor bugfix
Test: ToggleFeaturePreferenceFragmentTest
      (existing automated tests for content description)
Test: manually observe content description is still present
Change-Id: I9b163437de4941633d0af3b1d2ee6e679578fc68