Move utility classes in a separate static target

Split up utility classes that can be re-used in other IComposer related
tests (such as the IVrComposerClient tests).

Changes:
 * Move GraphicsComposerCallback into separate module
 * Extract TestCommandReader in separate module
 * Extract Composer::execute() from test class

Bug: 38203529
Test: Compiled
Change-Id: I0d1193469e9dd94c477a63058f98111c162dfa99