bionic: make property area expandable

bionic: make property area expandable

The property area is initially one 4K region, automatically expanding as
needed up to 64 regions.

To avoid duplicating code, __system_property_area_init() now allocates
and initializes the first region (previously it was allocated in init's
init_property_area() and initialized in bionic).  For testing purposes,
__system_property_set_filename() may be used to override the file used
to map in regions.

Change-Id: Ibe00ef52464bfa590953c4699a6d98383b0142b1
Signed-off-by: Greg Hackmann <ghackmann@google.com>