Support vendor override in default VHAL.

Move vendor override property logic from emulated VHAL to
default VHAL. If system property
"persist.vendor.vhal_init_value_override" is set, then we would
try to parse JSON files in "/vendor/etc/vhaloverride/" and use the
values there to override the default initial values.

Test: unit test
Bug: 193831021
Change-Id: I6823aab3de284d5b1d96186bc9f1325d868fd8d6