Update adb to android sdk relative path

[hsdk123]

This just makes it more convenient in the event that adb isn't in one's path.

[mosra]

Same as with #11, right now I can't reliably verify this doesn't break existing uses where adb might be in a different location, so I'll have to postpone this until I have a real computer again.

[hsdk123]

I should have switched branches when making this.. I'll close this for the moment and reopen later.

[mosra]

Finally merged the first commit (half-year delays, sorry).

Oh -- the rest is very cool. I'm interested in that also, so to avoid it getting lost/forgotten I'm reopening this PR. To make the change backwards-compatible I could imagine using cmake_parse_arguments() (which isn't in the minimal supported 3.4 yet, new in 3.5, but the Android toolchains requires 3.7 anyway, so that's okay) turning the function signature into for example (following the same format CMake is using in their docs):

android_create_apk(<target> <manifest> 
    [RESOURCES <res>] 
    [ASSETS <assets>])

If I get some time I can pick this up myself, just want to make sure this doesn't get forgotten :)

[mosra]

The remaining TODO here -- passing paths to resources and assets -- is finally implemented in b3bd787 and deccc40, including proper dependency tracking for any files in given directories that either change, are added or are removed. Usage is as follows, I'll update the docs once I have confirmed that everything works as intended:

android_create_apk(<target> <manifest> 
    [RESOURCE_DIRECTORY <res>] 
    [ASSET_DIRECTORY <assets>])