Dynamically add the webview_zygote's preloaded APK to the zygote FD whitelist.

This refactors the whitelist to be a class, rather than just a static C array.
The whitelist can then be augmented dynamically when the package path is known
in the webview_zygote.

Test: m
Test: sailfish boots
Test: Enable Multi-process WebView in developer options, perform a search in GSA.

Bug: 21643067
Change-Id: Ia1f2535c7275b42b309631b4fe7859c30cbf7309