Ensure mCurrentPage is valid after page is removed

Specific case this addresses: removing work profile
- Removing work profile causes workspace to be rebound
- This means pages are removed and re-added
- As icons are added back to the pages, they use the
  context of the current page to ensure theming is
  correct (see ag/1919636)
- Without this change, the current page could be null,
  causing a crash. Now we make sure the index is valid.

Bug: 62699220
Change-Id: I0aa1d0c2c6150c51dacb2eb860e4dacd183b3505