Add checks for types in Parcel / avoid class initialization

Make Parcel more stringent to avoid initializing classes
that are not related to Parcelable.

Two new checks:
1) That the class found on the stream implements Parcelable.
2) That the type of the CREATOR field declared on the class
found on the stream actually implements Parcelable.Creator.

For (1) the new check that a class in the stream is actually
Parcelable. This will affect handling of invalid streams or
code that didn't obey the requirements.

For (2) this change could break some apps that had a CREATOR
field in a Parcelable class that was not declared to be
(at least) a Parcelable.Creator: it is no longer sufficient
for the type to implement Parcelable.Creator, the field
must be declared as such.

This change includes doc updates for Parcelable to make
the requirement around the declared type of the CREATOR
field more concrete.

This change also makes the generics slightly tidier/explicit,
annotates code as unchecked where needed and removes some
assumptions that can not be guaranteed with Java's type
system and the current definitions.

For example, there's no guarantee right now that
Parcelable.Creator returns objects that are actually
Parcelable, or that the CREATOR object associated
with a Parcelable will return objects of the surrounding
class. The first we can't do something about without
breaking the public API (due to implementations like
TextUtils.CHAR_SEQUENCE_CREATOR). The second is
currently typically implicitly enforced with an implicit
cast in the (app's) calling code (e.g. callers to
readParcelable() that causes a language-introduced cast
to the type expected). A larger refactoring of Parcel
would be required to ensure that the class that is
produced by Creator is of a type compatible with the
class that declared CREATOR, and is not a goal for this
change.

A fix is included for a class that doesn't implement
Parcelable like it should and would probably fail
check (1).

Bug: 1171613
Change-Id: I31d07516efee29a320e80f4bc4f96aaac628f81c