Use kernel default for initial thread size

Bug: 10697851

Change-Id: I8d980f5e0b584799536f6e6b891056c968d26cdf
Simplify main thread stack size initialization

Change-Id: Iec09433d9de501031cce09dc75848a5e8f3d96bf
Merge "Use kernel default for initial thread size"
Merge "Simplify main thread stack size initialization"
Merge https://android.googlesource.com/platform/bionic into jb-devel-mr2-release