Credential management app UI tweaks

Credential management app Settings screen
* Add uninstall certificates button
* Reformat summary
* Remove dividing lines
Request manage credentials screen
* Make requesting app name bold

Manual testing
* Install TestDPC and select 'Request to
  manage credentials'
* Verify app name is bold and select allow
* Install a certificate as the cred mng app
  via TestDPC
* Go to Setting > Security > Encryption
  & Credentials > Credential Management App
* Verify screen looks like mocks
* Select 'Remove certificates' and
  verify all certificates installed by the
  cred mng app are removed
Settings screen: https://screenshot.googleplex.com/C6n9bnwkw843jgf
Request screen: https://screenshot.googleplex.com/5qH44NXKKGRsdHs

Bug: 189416800
Test: manual testing
      CredentialManagementAppControllerTest
      CredentialManagementAppFragmentTest
      CredentialManagementAppButtonsControllerTest
      CredentialManagementAppHeaderControllerTest
      RequestManageCredentialsTest
Change-Id: I71334b231ae7cefbc055e2575eda5418da2548b4