Fix android build

[JessyDL]

added build for android, fixed various compile errors related to android platform

Removed the following CMake settings:

• PE_PCH: was unused and untested for a while now
• PE_SOURCE_ONLY: lost its raison d'être a while ago

Added new define:
SURFACE_ANDROID, due to the different nature of the android surface this felt appropriate.

std::execution was wrapped in psl::ecs::execution (only place this was used). Android's NDK doesn't have it defined yet. Now comes with psl::ecs::execution::no_exec for when it's unsupported.

core::os::context added, empty on non-android systems, but on android has info when the app goes to the background (as well as window info). Considering to move to core::os::surface, but not sure yet.

Currently android builds are set up in the following way:
invoke python3 tools/android.py will set up the project files in builds/android. From there you can invoke ./gradlew bundle in builds/android. Output will appear in the expected location for an android project.

Alternatively setting PE_PLATFORM in cmake will generate the projects as well (internally by calling the previous script), two utility targets will be made android_paradigm_generate and android_paradigm_build (latter has dependency on the former)