Soong: Add relativeFilePathFlag category in CMakeList generator

* Add relativeFilePathFlag compiler flag category whose format must be
  a=b/c/d, where "a" is the flag name and "b/c/d" is the flag value that
  is a relative path
* During CMake conversion, "b/c/d" will be prefixed with ANDROID_ROOT
  before being written to CMakeLists.txt

Bug: 112599284
Test: cd system/bt; mm -j40
      croot; make -j40

Change-Id: I4bb039678482201db60f0af97ab280d5028e0463