Compare Pss of tested library in VerifyMemorySaving

DlExtRelroSharingTest.VerifyMemorySaving verifies whether
android_dlopen_ext(ANDROID_DLEXT_USE_RELRO) really saves memory.

This patch compares Pss of the tested library rather than total
Pss of the process, in case different behavior of dlopen() and
android_dlopen_ext() introduces random Pss usage.

Test: DlExtRelroSharingTest.VerifyMemorySaving

(cherry picked from commit 81aad005)

Bug: https://issuetracker.google.com/38226388
Change-Id: I4248628fa951b38818fe2af363fdf78abf09a112