Only write main library's RELRO by default.

ANDROID_DLEXT_WRITE_RELRO was inadvertently writing out the RELRO
section of all libraries loaded during a given dlopen() call instead of
only the main library; since the other libraries are loaded at
unpredictable addresses this additional data is rarely useful.

Fix this to only happen when the
ANDROID_DLEXT_RESERVED_ADDRESS_RECURSIVE flag is being used.

Bug: 128623590
Test: DlExtRelroSharingTest.CheckRelroSizes
Change-Id: I05e8651d06ce2de77b8c85fe2b6238f9c09691ad
(cherry picked from commit 5d10374947824c60d524e812172ae60f30a51fde)