Remove usages of now removed getThemedContext()

This was removed in FragmentActivity and so is blocking updating
AndroidX prebuilts. This method always returned 'this' so it's safe to
just use the Activity directly.

Bug: b/117613134
Test: run m -j
Change-Id: Iab8f69a68ffbdb58c7bc1246f3b92ec6aba9c97a