Split vendor's system prop overrides into vendor partition

If ENABLE_TREBLE and early mount of vendor partition are enabled,
overrides in PRODUCT_DEFAULT_PROPERTY_OVERRIDES will be stored in
vendor/default.prop.

If ENABLE_TREBLE and vendor partition are enabled, overrides in
PRODUCT_PROPERTY_OVERRIDES will be stored in vendor/build.prop.

default.prop of recovery image will include all the contents of
vendor/default.prop and vendor/build.prop additionally.

ENABLE_EARLY_MOUNT is added to specify whether early mount is enabled
or not, but it should be removed later when early mount becomes
default for all ENABLE_TREBLE products.

Test: building succeeded, and verified on bullhead.
Bug: 34116668
Change-Id: Ic2eb8882ae71921a76c02b3d62d3289c45949295