Parse /proc/pid/status for memory only once

Read and parse /proc/pid/status for native processes only once.
Previously, it was read twice (getUidForPid and read*FromProcfs) and
regular expressions were applied to its contents few times.

A single read also allowed to simplify the code: use null to represent
failed read.

Bug: 140986627
Test: atest ProcfsMemoryUtilTest
Test: atest UidAtomTests#testProcessMemorySnapshot
Test: atest UidAtomTests#testProcessMemoryHighWaterMark
Test: atest UidAtomTests#testNativeProcessMemoryState
Change-Id: I09d05b663f99602fcfacae253af7349989d48749