[ASan] Change activation strategy.

[ASan] Change activation strategy.

Now ASan deactivation doesn't modify common or ASan-specific runtime
flags. Flags stay constant after initialization, and "deactivation"
instead stashes initialized runtime state, and deactivates the
runtime. Activation then just restores the original state (possibly,
overriden by some activation flags provided in system property on
Android).

llvm-svn: 224614