Fix: convert number of pages to bytes

According to man proc(5), the 24th field in /proc/pid/stat
is given in the number of pages (not bytes). Convert it to
bytes by multiplying by the default page size in Android.

Also, replace Long.valueOf with Long.parseLong to avoid
auto-unboxing and wrap parsing in a try-catch where need.

Change-Id: Ifb20120019bd962b6e8ce5b69d16853218f9575b
Fix: 115639792
Test: atest MemoryStatUtilTest