Store WebView package signatures as Signatures instead of Strings

We are currently storing WebView package signatures as Strings in
WebViewProviderInfo, represent these as Signatures instead.

Bug: 62429931
Test: start WebView app + run WebViewTest GTS tests + WVUS tests
Change-Id: I910db133cd147b44b4b1c63921308d82db44e5b3
3 files changed