Support loading libraries to a reserved address.

Support loading libraries to a reserved address.

Add flags and parameters to android_dlopen_ext() to allow loading a
library at an already-reserved fixed address. If the library to be
loaded will not fit within the space reserved, then the linker will
either fail, or allocate its own address space as usual, according to
which flag has been specified. This behaviour only applies to the
specific library requested; any other libraries loaded as dependencies
will be loaded in the normal fashion.

There is a new gtest included to cover the functionality added.

Bug: 13005501
Change-Id: I5d1810375b20fc51ba6a9b3191a25f9792c687f1