Fix search indexing for encryption_and_credential page

- Rewrite search index provider to include all possible keys through
  getXmlResourcesToIndex()
- Add isPageSearchEnabled() to disable all keys if user is not admin
- Add getNonIndexableKeys to suppress unrelated keys based on current
  device state

Change-Id: I2c6943483789bf4c9f3931d344cf279fec0edaee
Fix: 37650170
Test: robotests