Keystore 2.0 SPI: Zygote install Keystore2 provider conditionally

This patch makes Zygote install the Keystore 2.0 SPI as
"AndroidKeyStore" and the old Keystore SPI as "AndroidKeyStoreLegacy"
if the platform property ro.android.security.keystore2.enable is
set to true. This allows us to boot Android with vital components, such
as LockSettingsService, still using the legacy Keystore, while we run
CTS tests against the new implementation, migrate other system
components, and perform migration tests.

This CL will be superseded by a CL that makes Zygote install the
Keystore 2.0 SPI exclusively when the migration of all dependent
components is complete.

Bug: 171305684
Test: None
Change-Id: I9e32578285167c4d63f4f536a07fe98473a883e0