Add flag to "am start" to enable native debugging

Specifying the new flag will enable several features in the runtime
required by the native debugger to debug Java and C++ code at the same
time.

The enabled features:
* Force JIT (never use the interpreter)
* Debug info generation
* Disbale some optimizations

Change-Id: Iaf5ab649715a0c274bd1b0fc64e483705da53cd0