[build] Reimplement NdkUtils

[grendello]

Context: #5996
Context: #5964
Context: #5964 (comment)

The NdkUtils class used by Xamarin.Andrid.Build.Tasks to find tooling
shipped with the Android NDK, has grown increasingly complicated over
the years due to a number of incompatibilities between various versions
of the NDK. The code became hard to follow and untidy. This commit
attempts to address the issue by replacing the single static NdkUtils
class with a hierarchy of dynamically instantiated classes rooted in a
new base class, NdkTools.

NdkUtils had to be initialized for each thread that needed to access
its methods, which led to various issues with concurrency and lack of
proper initialization since the initialization had to be done wherever
NdkUtils was first accessed, meaning that any task using it had to do
it.

NdkTools doesn't require such initialization, instead it provides a
factory method called Create which takes path to the NDK as its
parameter and returns an instance of NdkTools child class (or null
if an error occurs) which the can be safely used by the caller. Callers
need not concern themselves with what is the actual type of the returned
instance, they access only methods and properties defined in the
NdkTools base abstract class.

The hierarchy of NdkTools derivatives is structured and named after
the breaking changes in the NDK. For instance, NDK versions before 16
used the GNU compilers, while release 16 and above use the clang
compilers - this is reflected in existence of two classes derived from
NdkTools, NoClang for NDKs older than r16 and WithClang for the
newer ones. The other breaking changes are the addition of unified
headers in r19, removal of the platforms directory in r22 and removal
of GNU Binutils in r23.

NDK r23 is recognized in this commit but it is NOT supported. Support
for r23 is being worked on in PR #6073 which will be merged once r23 is
out of beta.

[jonpryor]

Context: #5996
Context: #5964
Context: #5964 (comment)