ANDROID: 16K: [s]maps: Fold fixup entries into the parent entry

In x86_64 16kb emulation mode the kernel will insert anonymous VMAs
at the end of filebacked of shmem mappings that are larger than the
backing file. This is done inorder to avoid invalid file faults.

The smaps/maps entries for fixup VMAs are currently hidden from
userspace to prevent processes performing syscalls on non-page
aligned addresses by scanning /proc/<pid>/[s]maps. However the end
of the original VMA is left as a non-16kB aligned address.

Fold the extend of the fixup VMA into the parent (preceding VMA).
This hides fixup VMA completely from userspace and any operations
performed with addresses from maps or smaps will be done on the
orignal range.

Before:

74b028b48000-74b028b49000 r--p 00000000 00:12 1441    /dev/test.so
74b028b4c000-74b028b50000 rw-p 00000000 00:00 0       [anon:Test mapping]

The unaligned gap [74b028b49000-74b028b4c000] between the 2 test
mappings is where the anon fixup was inserted.

After:

74b028b48000-74b028b4c000 r--p 00000000 00:12 1441    /dev/test.so
74b028b4c000-74b028b50000 rw-p 00000000 00:00 0       [anon:Test mapping]

There is no visible gap and the fixup mapping is now transparent to
userpsace.

Bug: 365566776
Bug: 365177458
Bug: 328777915
Bug: 331683943
Change-Id: Ibe2fe630695d875a8d7798f235adcf277b16bd9c
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
(cherry picked from commit 77e90f43)