Fix Soong CMakeLists.txt generator

Problem: Soong does not guarantee that the list of parameters will
be in individual strings. This means the CMakeLists generator can
receive as input:

  params = {"-isystem", "path/to/system"}

or it could receive:

  params = {"-isystem path/to/system"}

Solution: Normalize the list of parameters into a flattened list
of parameters where each parameters is in an individual string.

Fixes: 143378093
Test: None
Change-Id: I9bd1a2006a1cf0ba383f467748b6fd4eadef6866