Convert enableGwpAsan to @IntDef and rename to gwpAsanMode.

Following API feedback, make
  ApplicationInfo.enableGwpAsan
  ProcessInfo.enableGwpAsan
a @IntDef integers instead of @Nullable booleans.

Rename both, and the manifest attribute, to gwpAsanMode.
Turn the manifest attribute into an enum.
Fix manifest documentation to match the implementation.

Bug: 151262688
Test: CtsGwpAsanTestCases
Change-Id: Ie33954700e1526f9764a751e474a3f930a65e957