Register environment in all bean factories in a hierarchy

Prior to this change, AbstractApplicationContext#prepareBeanFactory
registered a bean named 'environment' once and only once within a given
ApplicationContext hierarchy. This worked fine with the expectation
that the Environment object is always delegated downward to children of
that hierarchy. However, with SPR-9444 and the introduction of
ConfigurableEnvironment#merge, this expectation was violated; each
member of an application context hierarchy now maintains its own
distinct Environment instance, which means that by extension that each
application context's underlying BeanFactory should have its own
'environment' bean pointing to that context's environment instance.

This problem could manifest in getting the wrong environment instance
when calling #getBean(Environment) or when @Autowiring an Environment
instance, for example into a @Configuration class. As reported in
SPR-9756, this could result in false negative property lookups or
incorrect results when checking whether a profile is active.

This commit ensures that every bean factory in an application
hierarchy has an 'environment' bean referring to the object returned
from the enclosing ApplicationContext#getEnvironment method.

Issue: SPR-9756, SPR-9444
Remove default profile during environment merge

This change fixes a minor bug with the implementation of
ConfigurableEnvironment#merge, introduced in SPR-9444. During a merge
of two environments A and B, where A has default profiles [prod] and B
has default profiles [default] (the so-called 'reserved default
profile'), B would complete the merge process having a collection of
profiles reading [default, prod], which is incorrect.

This commit explicitly ensure's that B's reserved default profile is
removed if A has a set of default profiles greater than zero. If A
consists only of [default], B will inherit it during the merge
correctly; if A consists of [p1, p2], B will result in [p1, p2] as
well; if B consists of [p1] and A of [p2, p3], B will result in
[p1, p2, p3] post-merge.

Issue: SPR-9761, SPR-9444
Refactor to lazy Environment creation where possible

This commit avoids eager creation of Environment instances, favoring
delegation of already existing Environment objects where possible. For
example, FrameworkServlet creates an ApplicationContext; both require
a StandardServletEnvironment instance, and prior to this change, two
instances were created where one would suffice - indeed these two
instances may reasonably be expected to be the same. Now, the
FrameworkServlet defers creation of its Environment, allowing users to
supply a custom instance via its #setEnvironment method (e.g. within a
WebApplicationInitializer); the FrameworkServlet then takes care to
delegate that instance to the ApplicationContext created
in #createWebApplicationContext.

This behavior produces more consistent behavior with regard to
delegation of the environment, saves unnecessary cycles by avoiding
needless instantiation and calls to methods like
StandardServletEnvironment#initPropertySources and leads to better
logging output, as the user sees only one Environment created and
initialized when working with the FrameworkServlet/DispatcherServlet.

This commit also mirrors these changes across the corresponding
Portlet* classes.

Issue: SPR-9763